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"Web 2.0, according to 2005 conference sponsor Tim O'Reilly, is an "architecture of participation" -- a constellation made up of links between web applications that rival desktop applications, the blog publishing revolution and self-service advertising. This architecture is based on social software where users generate content, rather than simply consume it, and on open programming interfaces that let developers add to a web service or get at data. It is an arena where the web rather than the desktop is the dominant platform, and organization appears spontaneously through the actions of the group, for example, in the creation of folksonomies created through tagging."

The theory has been percolating for some time. But it intensified last year when O'Reilly published an essay on the topic, as well as a graphic outlining the key categories of this new medium.