Though people disagree on what Web2.0 really is. Initially it was just the name of a conference created by O’Reilly Publishers. The term then snowballed to embrace UGC, Social Networking, Ajax, Folksonomies (user tagging) and other stuff that is now commonplace. At the beginning I think it was simply a way for clued up web people to encapsulate all that was new and exciting about the web. Now of course it’s been hijacked by marketing types to flog any old service.
The idea of a semantic web predates Web2.0 as a concept and has been promoted by Tim_Berners-Lee (the creator of the WWW) for a while now. Calling it Web3.0 is stupid but, alas, probably inevitable.
I think naming web stages in advance is a mugs game. Only from looking at the history will we really see any “stages” and pinpoint the moments that truely changed the game. Maybe it will be more closely related to the economics of the market rather than the technologies; if there’s another burst bubble, that’s what will be more significant, not that someone worked out how to do chat over AJAX.