The term became notable after the first 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 to changes in the ways software developers and end-users use the Web. According to Tim O'Reilly:

Some technology experts, notably Tim Berners-Lee, have questioned whether one can use the term in a meaningful way, since many of the technology components of "Web 2.0" have existed since the early days of the Web.

Characteristics

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 very old traditional websites, the sort which 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, openlaszlo, Flex or similar rich media. The sites may also have social-networking aspects.

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.

According to Best, the characteristics of Web 2.0 are: rich user experience, user participation, dynamic content, metadata, web standards and scalability. Three further characteristics that Best did not mention about web 2.0: openness, freedom and collective intelligence by way of user participation – all should be viewed as essential attributes of Web 2.0.

Technology Overview

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:

• Cascading Style Sheets to aid in the separation of presentation and content

• Folksonomies (collaborative tagging, social classification, social indexing, and social tagging)

• Microformats extending pages with additional semantics

• REST and/or XML- and/or JSON-based APIs

• Rich Internet application techniques, often Ajax and/or Flex,Flash-based

• Semantically valid XHTML and HTML markup

• Syndication, aggregation and notification of data in RSS or Atom feeds

• mashups, merging content from different sources, client- and server-side

• Weblog-publishing tools

• wiki or forum software, etc., to support user-generated content

• Internet privacy, the extended power of users to manage their own privacy in cloaking or deleting their own user content or profiles.