Web 2.0 (also known as participative, participatory, web and social web) refers to websites that emphasize user-generated content, ease of use, participatory culture, and interoperability (i.e., compatibility with other products, systems, and devices) for end users.

The term was coined by Darcy DiNucci in 1999 and later popularized by Tim O'Reilly and Dale Dougherty at the first Web 2.0 Conference in 2004. Although the term mimics the numbering of software versions, it does not denote a formal change in the nature of the World Wide Web; the term merely describes a general change that occurred during this period as interactive websites proliferated and came to overshadow the older, more static websites of the original Web.

A Web 2.0 website allows users to interact and collaborate through social media dialogue as creators of user-generated content in a virtual community. This contrasts the first generation of Web 1.0-era websites where people were limited to passively viewing content. Examples of Web 2.0 features include social networking sites or social media sites (e.g., Facebook), blogs, wikis, folksonomies ("tagging" keywords on websites and links), video sharing sites (e.g., YouTube), image sharing sites (e.g., Flickr), hosted services, Web applications ("apps"), collaborative consumption platforms, and mashup applications.

Web 1.0 is a retronym referring to the first stage of the World Wide Web's evolution, from roughly 1989 to 2004. According to Graham Cormode and Balachander Krishnamurthy, "content creators were few in Web 1.0 with the vast majority of users simply acting as consumers of content". Personal web pages were common, consisting mainly of static pages hosted on Internet service provider-run web servers, or on free web hosting services such as the now-defunct Tripod and GeoCities.

With Web 2.0, it became common for average web users to have social-networking profiles (on sites such as Myspace and Facebook) and personal blogs (sites like Blogger, Tumblr and LiveJournal) through either a low-cost web hosting service or through a dedicated host. In general, content was generated dynamically, allowing readers to comment directly on pages in a way that was not common previously.

Some Web 2.0 capabilities were present in the days of Web 1.0, but were implemented differently. For example, a Web 1.0 site may have had a guestbook page for visitor comments, instead of a comment section at the end of each page (typical of Web 2.0). During Web 1.0, server performance and bandwidth had to be considered—lengthy comment threads on multiple pages could potentially slow down an entire site. Terry Flew, in his third edition of New Media, described the differences between Web 1.0 and Web 2.0 as a

Flew believed these factors formed the trends that resulted in the onset of the Web 2.0 "craze".

Writing when Palm Inc. introduced its first web-capable personal digital assistant (supporting Web access with WAP), DiNucci saw the Web "fragmenting" into a future that extended beyond the browser/PC combination it was identified with. She focused on how the basic information structure and hyper-linking mechanism introduced by HTTP would be used by a variety of devices and platforms. As such, her "2.0" designation refers to the next version of the Web that does not directly relate to the term's current use.

The term Web 2.0 did not resurface until 2002. Companies such as Amazon, Facebook, Twitter, and Google, made it easy to connect and engage in online transactions. Web 2.0 introduced new features, such as multimedia content and interactive web applications, which mainly consisted of two-dimensional screens. Kinsley and Eric 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 to popularize when O'Reilly Media and MediaLive hosted the first Web 2.0 conference.

In their opening remarks, John Battelle 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". They argued that the activities of users generating content (in the form of ideas, text, videos, or pictures) could be "harnessed" to create value. O'Reilly and Battelle contrasted Web 2.0 with what they called "Web 1.0".

They associated this term with the business models of Netscape and the Encyclopædia Britannica Online. For example,

The popularity of Web 2.0 was acknowledged by 2006 TIME magazine Person of The Year (You). That is, TIME recognized the masses of users who were participating in content creation on social networks, blogs, wikis, and media sharing sites.

Instead of merely reading a Web 2.0 site, a user is invited to contribute to the site's content by commenting on published articles, or creating a user account or profile on the site, which may enable increased participation. By increasing emphasis on these already-extant capabilities, they encourage users to rely more on their browser for user interface, application software ("apps") and file storage facilities. This has been called "network as platform" computing.

Major features of Web 2.0 include social networking websites, self-publishing platforms (e.g., WordPress' easy-to-use blog and website creation tools), "tagging" (which enables users to label websites, videos or photos in some fashion), "like" buttons (which enable a user to indicate that they are pleased by online content), and social bookmarking.

Users can provide the data and exercise some control over what they share on a Web 2.0 site. These sites may have an "architecture of participation" that encourages users to add value to the application as they use it. Users can add value in many ways, such as uploading their own content on blogs, consumer-evaluation platforms (e.g. Amazon and eBay), news websites (e.g. responding in the comment section), social networking services, media-sharing websites (e.g. YouTube and Instagram) and collaborative-writing projects.

Some scholars argue that cloud computing is an example of Web 2.0 because it is simply an implication of computing on the Internet.

Web 2.0 offers almost all users the same freedom to contribute, which can lead to effects that are varyingly perceived as productive by members of a given community or not, which can lead to emotional distress and disagreement. The impossibility of excluding group members who do not contribute to the provision of goods (i.e., to the creation of a user-generated website) from sharing the benefits (of using the website) gives rise to the possibility that serious members will prefer to withhold their contribution of effort and "free ride" on the contributions of others.

This requires what is sometimes called radical trust by the management of the Web site.

The client-side (Web browser) technologies used in Web 2.0 development include Ajax and JavaScript frameworks. The term "Ajax" was coined by Jesse James Garrett in a 2005 essay describing it as a combination of existing technologies, HTML/CSS, the DOM, and the XMLHttpRequest object, rather than a single new technology. Ajax programming uses JavaScript and the Document Object Model (DOM) to update selected regions of the page area without undergoing a full page reload.

To allow users to continue interacting with the page, communications such as data requests going to the server are separated from data coming back to the page (asynchronously).

Otherwise, the user would have to routinely wait for the data to come back before they can do anything else on that page, just as a user has to wait for a page to complete the reload. This also increases the overall performance of the site, as the sending of requests can complete quicker independent of blocking and queueing required to send data back to the client. 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 to dynamically update the Web page based on the new data, allowing for rapid and interactive user experience. In short, using these techniques, web designers can make their pages function like desktop applications. For example, Google Docs uses this technique to create a Web-based word processor.