Characteristics of "Web 2.0"
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, 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.
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:
rich Internet application techniques, often Ajax-based
semantically valid XHTML and HTML markup
microformats extending pages with additional semantics
folksonomies (in the form of tags or tagclouds, for example)
Cascading Style Sheets to aid in the separation of presentation and content
REST and/or XML- and/or JSON-based APIs
syndication, aggregation and notification of data in RSS or Atom feeds
mashups, merging content from different sources, client- and server-side
wiki or forum software, etc., to support user-generated content
Innovations sometimes associated with "Web 2.0"
Web-based applications and desktops
The richer user-experience afforded by Ajax has prompted the development of websites that mimic personal computer 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 been developed, including EyeOS and YouOS. They essentially function as application platforms, not as operating systems per se. These services mimic the user experience of desktop operating-systems, offering features and applications similar to a PC environment. They have as their distinguishing characteristic the ability to run within any modern browser.
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 USD45 million.
Rich Internet applications
Recently, rich-Internet application techniques such as Ajax, Adobe Flash, Flex, and Silverlight have evolved that have the potential to improve the user-experience in browser-based applications. These technologies allow a web-page to request an update for some part of its content, and to alter that part in the browser, without needing to refresh the whole page at the same time.
Functionally, Web 2.0 applications build on the existing Web server architecture, but rely much more heavily on back-end software. Syndication differs only nominally from the methods of publishing using dynamic content management, but web services typically require much more robust database and workflow support, and become very similar to the traditional intranet functionality of an application server. Vendor approaches to date fall either under a universal server approach (which bundles most of the necessary functionality in a single server platform) or under a web-server plugin approach (which uses standard publishing tools enhanced with API interfaces and other tools).
XML and RSS
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.
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.
REST (Representational State Transfer) Web APIs use HTTP alone to interact, with XML or JSON payloads;
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 (eXtensible Markup Language) 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.