"Web 2.0" refers to the second generation of web development and web design that facilitates information sharing, interoperability, user-centered design and collaboration on the World Wide Web. The advent of Web 2.0 led to the development and evolution of web-based communities, hosted services, and web applications. Examples include social-networking sites, video-sharing sites, wikis, blogs, mashups and folksonomies.

The term is now closely associated with Tim O'Reilly because of the O'Reilly Media Web 2.0 conference in 2004. Although the term suggests a new version of the World Wide Web, it does not refer to an update to any technical specifications, but rather to cumulative changes in the ways software developers and end-users use the Web. Whether Web 2.0 is qualitatively different from prior web technologies has been challenged by World Wide Web inventor Tim Berners-Lee who called the term a "piece of jargon".

The term "Web 2.0" was coined by Darcy DiNucci in 1999. In her article "Fragmented Future," she writes

The Web we know now, which loads into a browser window in essentially static screenfuls, is only an embryo of the Web to come. The first glimmerings of Web 2.0 are beginning to appear, and we are just starting to see how that embryo might develop The Web will be understood not as screenfuls of text and graphics but as a transport mechanism, the ether through which interactivity happens. It will [...] appear on your computer screen, on your TV set your car dashboard your cell phone hand-held game machines and maybe even your microwave.

Her use of the term deals mainly with Web design and aesthetics; she argues that the Web is "fragmenting" due to the widespread use of portable Web-ready devices. Her article is aimed at designers, reminding them to code for an ever-increasing variety of hardware. As such, her use of the term hints at - but does not directly relate to - the current uses of the term.

The term did not resurface until 2003. These authors focus on the concepts currently associated with the term where, as Scott Dietzen puts it, "the Web becomes a universal, standards-based integration platform."

In 2004, the term began its rise in popularity when O'Reilly Media and MediaLive hosted the first Web 2.0 conference. In their opening remarks, John Batelle and Tim O'Reilly outlined their definition of the "Web as Platform," where software applications are built upon the Web as opposed to upon the desktop. The unique aspect of this migration, they argued, is that "customers are building your business for you."That is, they argued that the activities of users generating content (in the form of ideas, text, videos, or pictures) could be "harnessed" to create value. According to Tim O'Reilly:

Web 2.0 is the business revolution in the computer industry caused by the move to the Internet as a platform, and an attempt to understand the rules for success on that new platform.

From there, the term Web 2.0 was largely championed by bloggers and by technology journalists, culminating in the 2006 TIME magazine Person of The Year - "You." That is, TIME selected the masses of users who were participating in content creation on social networks, blogs, wikis, and media sharing sites. The cover story author Lev Grossman explains:

It's a story about community and collaboration on a scale never seen before. It's about the cosmic compendium of knowledge Wikipedia and the million-channel people's network YouTube and the online metropolis MySpace. It's about the many wresting power from the few and helping one another for nothing and how that will not only change the world, but also change the way the world changes.

Web 2.0 websites allow users to do more than just retrieve information. They can build on the interactive facilities of "Web 1.0" to provide "Network as platform" computing, allowing users to run software-applications entirely through a browser.Users can own the data on a Web 2.0 site and exercise control over that data. These sites may have an "Architecture of participation" that encourages users to add value to the application as they use it.This stands in contrast to traditional websites, the sort that limited visitors to viewing and whose content only the site's owner could modify. Web 2.0 sites often feature a rich, user-friendly interface based on Ajax[2][3] and similar client-side interactivity frameworks, or full client-server application frameworks such as OpenLaszlo, Flex, and the ZK framework..

The concept of Web-as-participation-platform captures many of these characteristics. Bart Decrem, a founder and former CEO of Flock, calls Web 2.0 the "participatory Web"and regards the Web-as-information-source as Web 1.0.

The impossibility of excluding group-members who don’t contribute to the provision of goods from sharing profits gives rise to the possibility that rational members will prefer to withhold their contribution of effort and free-ride on the contribution of others.This requires what is sometimes called Radical Trust by the management of the website. According to Best,the characteristics of Web 2.0 are: rich user experience, user participation, dynamic content, metadata, web standards and scalability. Further characteristics, such as openness, freedom and collective intelligence by way of user participation, can also be viewed as essential attributes of Web 2.0

The sometimes complex and continually evolving technology infrastructure of Web 2.0 includes server-software, content-syndication, messaging-protocols, standards-oriented browsers with plugins and extensions, and various client-applications. The differing, yet complementary approaches of such elements provide Web 2.0 sites with information-storage, creation, and dissemination challenges and capabilities that go beyond what the public formerly expected in the environment of the so-called "Web 1.0". Web 2.0 websites typically include some of the following features/techniques. Andrew McAfee used the acronym SLATES to refer to them:

The ability to create constantly updating content over a platform that is shifted from being the creation of a few to being constantly updated, interlinked work. In wikis, the content is iterative in the sense that users undo and redo each other's work. In blogs, content is cumulative in that posts and comments of individuals are accumulated over time.

Categorization of content by creating tags: simple, one-word user-determined descriptions to facilitate searching and avoid rigid, pre-made categories.

Powerful algorithms that leverage the Web as an application platform as well as a document server.

The client-side/web browser technologies typically used in Web 2.0 development are Asynchronous JavaScript and XML (Ajax), Adobe Flash, and Javascript/Ajax frameworks such as YUI (Yahoo User Interface Library), Dojo, Mootools, and jQuery. Ajax programming uses JavaScript to upload and download new data from the web server without undergoing a full page reload.

The data fetched by an Ajax request is typically formatted in XML or JSON (Javascript Object Notation) format, two widely used structured data formats. Since both of these formats are natively understood by Javascript, a programmer can easily use them to transmit structured data in their web application. When this data is received via Ajax, the Javascript program then uses the Document Object Model (DOM) to dynamically update the web page based on the new data, allowing for a rapid and interactive user experience.

Adobe Flash is another technology often used in Web 2.0 applications. As a widely available plugin independent of W3C (World Web Consortium, the governing body of web standards and protocols), standards, Flash is capable of doing many things which are not currently possible in HTML, the language used to construct web pages. Of Flash's many capabilities, the most commonly used in Web 2.0 is its ability to play audio and video files. This fact alone has allowed for the creation of cutting edge Web 2.0 sites such as Youtube, where rich media is gracefully integrated with standard HTML.

In addition to Flash and Ajax, Javascript/Ajax frameworks have recently become a very popular means of creating Web 2.0 sites. At their core, these frameworks do not use technology any different from Javascript, Ajax, and the DOM. What frameworks do is smooth over inconsistencies between web browsers and extend the functionality available to developers. Many of them also come with customizable, prefabricated 'widgets' that accomplish such common tasks as picking a date from a calendar, displaying a data chart, making a tabbed panel, etc.

On the server side, Web 2.0 uses many of the same technologies as Web 1.0. Languages such as PHP, Perl, Python, and ASP are used by developers to dynamically output data using information from files and databases. What has begun to change in Web 2.0 is the way this data is formatted. In the early days of the internet, there was little need for different websites to communicate with each other and share data. In the new 'participatory web', however, sharing data between sites has become an essential capability. To share its data with other sites, a web site must be able to generate output in machine-readable formats such as XML, RSS, and JSON. When a site's data is available in one of these formats, another website can use it to integrate a portion of that site's functionality into itself, linking the two together. When this design pattern is implemented, it ultimately leads to data that is both easier to find and more thoroughly categorized, a hallmark of the philosophy behind the Web 2.0 movement.

The popularity of the term Web 2.0, along with the increasing use of blogs, wikis, and social networking technologies, has led many in academia and business to coin a flurry of 2.0s,[20] including Library 2.0, Social Work 2.0,Enterprise 2.0, PR 2.0,Classroom 2.0, Publishing 2.0, Medicine 2.0, Travel 2.0 and Government 2.0.Many of these 2.0s refer to Web 2.0 technologies as the source of the new version in their respective disciplines and areas. For example, in the Talis white paper "Library 2.0: The Challenge of Disruptive Innovation," Paul Miller argues

Blogs, wikis and RSS are often held up as exemplary manifestations of Web 2.0. A reader of a blog or a wiki is provided with tools to add a comment or even, in the case of the wiki, to edit the content. This is what we call the Read/Write web.Talis believes that Library 2.0 means harnessing this type of participation so that libraries can benefit from increasingly rich collaborative cataloguing efforts, such as including contributions from partner libraries as well as adding rich enhancements, such as book jackets or movie files, to records from publishers and others.

Here, Miller links Web 2.0 technologies and the culture of participation that they engender to the field of library science, supporting his claim that there is now a "Library 2.0." Many of the other proponents of new 2.0s mentioned here use similar methods.

According to the Global Language Monitor, Web 2.0 is the one millionth word to enter the English language.

Ajax has prompted the development of websites that mimic desktop applications, such as word processing, the spreadsheet, and slide-show presentation. WYSIWYG wiki sites replicate many features of PC authoring applications. Still other sites perform collaboration and project management functions. In 2006 Google, Inc. acquired one of the best-known sites of this broad class, Writely.

Several browser-based "operating systems" have emerged, including EyeOS and YouOS. Although coined as such, many of these services function less like a traditional operating system and more as an application platform. They mimic the user experience of desktop operating-systems, offering features and applications similar to a PC environment, as well as the added ability of being able to run within any modern browser. However, these operating systems do not control the hardware on the client's computer.

Numerous web-based application services appeared during the dot-com bubble of 1997–2001 and then vanished, having failed to gain a critical mass of customers. In 2005, WebEx acquired one of the better-known of these, Intranets.com, for $45 million.

Advocates of "Web 2.0" may regard syndication of site content as a Web 2.0 feature, involving as it does standardized protocols, which permit end-users to make use of a site's data in another context (such as another website, a browser plugin, or a separate desktop application). Protocols which permit syndication include RSS (Really Simple Syndication — also known as "web syndication"), RDF (as in RSS 1.1), and Atom, all of them XML-based formats. Observers have started to refer to these technologies as "Web feed" as the usability of Web 2.0 evolves and the more user-friendly Feeds icon supplants the RSS icon.

Specialized protocols such as FOAF and XFN (both for social networking) extend the functionality of sites or permit end-users to interact without centralized websites.

Other protocols, like Jabber enables services to users like Services over the Messenger

Machine-based interaction, a common feature of Web 2.0 sites, uses two main approaches to Web APIs, which allow web-based access to data and functions: REST and SOAP

1. REST (Representational State Transfer) Web APIs use HTTP alone to interact, with XML (eXtensible Markup Language) or JSON payloads
2. SOAP involves POSTing more elaborate XML messages and requests to a server that may contain quite complex, but pre-defined, instructions for the server to follow.

Often servers use proprietary APIs, but standard APIs (for example, for posting to a blog or notifying a blog update) have also come into wide use. Most communications through APIs involve XML or JSON payloads. See also Web Services Description Language (WSDL) (the standard way of publishing a SOAP API) and this list of Web Service specifications.

The criticism exists that "Web 2.0" does not represent a new version of the World Wide Web at all, but merely continues to use so-called "Web 1.0" technologies and concepts. Techniques such as AJAX do not replace underlying protocols like HTTP, but add an additional layer of abstraction on top of them. Many of the ideas of Web 2.0 had already been featured in implementations on networked systems well before the term "Web 2.0" emerged. Amazon.com, for instance, has allowed users to write reviews and consumer guides since its launch in 1995, in a form of self-publishing. Amazon also opened its API to outside developers in 2002. Previous developments also came from research in computer-supported collaborative learning and computer-supported cooperative work and from established products like Lotus Notes and Lotus Domino.

In a podcast interview, Tim Berners-Lee described the term "Web 2.0" as a "piece of jargon":

"Nobody really knows what it means...If Web 2.0 for you is blogs and wikis, then that is people to people. But that was what the Web was supposed to be all along."

Other criticism has included the term “a second bubble” (referring to the Dot-com bubble of circa 1995–2001), suggesting that too many Web 2.0 companies attempt to develop the same product with a lack of business models. The Economist has also written about "Bubble 2.0".Venture capitalist Josh Kopelman noted that Web 2.0 had excited only 53,651 people (the number of subscribers at that time to TechCrunch, a Weblog covering Web 2.0 startups and technology news), too few users to make them an economically viable target for consumer applications.Although Bruce Sterling reports he's a fan of Web 2.0, he thinks it is now dead as a rallying concept.

Critics have cited the language used to describe the hype cycle of Web 2.0 as an example of Techno-utopianist rhetoric.

Critics such as Andrew Keen argue that Web 2.0 has created a cult of digital narcissism and amateurism, which undermines the notion of expertise by allowing anybody, anywhere to share (and place undue value upon) their own opinions about any subject and post any kind of content regardless of their particular talents, knowledgeability, credentials, biases or possible hidden agendas. He states that the core assumption of Web 2.0, that all opinions and user-generated content are equally valuable and relevant is misguided, and is instead "creating an endless digital forest of mediocrity: uninformed political commentary, unseemly home videos, embarrassingly amateurish music, unreadable poems, essays and novels," also stating that Wikipedia is full of "mistakes, half truths and misunderstandings".

In November 2004, CMP Media applied to the USPTO for a service mark on the use of the term "WEB 2.0" for live events.On the basis of this application, CMP Media sent a cease-and-desist demand to the Irish non-profit organization IT@Cork on May 24, 2006, but retracted it two days later.The "WEB 2.0" service mark registration passed final PTO Examining Attorney review on May 10, 2006, and was registered on June 27, 2006.[38] The European Union application (application number 004972212, which would confer unambiguous status in Ireland) remains currently[update] pending after its filing on March 23, 2006.