My definition of Web 2.0: comments please.
Web 2.0 is a marketing-driven buzzword to describe the way in which the Internet (or more specifically the World Wide Web) has become collaborative. That is, whereas in the past content was generally published and remained static for download by people for informational purposes, it has developed into a system whereby the user can contribute to the website in a fundamental way. The best examples of these are Wikis, openly editable encyclopedias where user-contributed content fuels the entire medium, blogs to which anybody can post a comment and Flikr which is a photo-sharing site. The entire purpose of these sites is to collect end-user input and display it in an organised fashion. There was no sudden 'change to Web 2.0' and it isn't a technological change per se, rather a culmination of years of development where websites have gone from static pages of content, through to dynamic online shops, catalogues and forums and finally to this point at which website visitors are often as responsible for generating the website content as the website owner.