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There is no standard definition for web 2.0, as it is a cluster of ideas rather than anything clear-cut. However, O'Reilly's comments on the topic are seen as having special authority, and rank among the top Google search results for the term.
The first premise of web 2.0 is leveraging the power of the user. For example, fluid user tagging of content would be used instead of a centralized taxonomy. Web 2.0 entrepreneurs often consider the Long Tail, which is basically an observation that the vast majority of the attention market is based on niche content. This version of the web is radically decentralized, as in the case of BitTorrent, a collaborative downloading co-op that consumes a serious portion of all Internet traffic.
Blogs are considered web 2.0. Instead of centralized "personal home pages", blogs let people easily post as much or as little as they want as rarely or as frequently as they want. Feed aggregators ensure that people only need to visit a single site to see all the feeds they subscribe to. Comments are enabled everywhere, allowing people to participate rather than passively consume content.
The web page Digg is an example of web 2.0. Unlike traditional news pages for which editors choose the top stories, Digg's front page content is determined by the voting of many thousands of users. The more votes a story gets, the more likely it is to be featured on the front page.
Web 2.0 marketing is supposed to be viral - that is, happy users encouraging their friends to use a product, rather than massive advertising brainwashing people into doing so. This ties in with the idea of "permission marketing" - marketing that actually gets the permission of its targets rather than shoving an ad in someone's face against their will. Some people call web 2.0 just another bubble like the first. Only time will tell whether these companies are truly profitable or "merely" trendy and useful.