Well Web 2.0 when coined by Tim O’Reilly at 2004 web conference was to
descibe "an attitude rather than a technology", so there is no physical web2.0 or web 3.0. Web 2.0 is mainly about user generated content like social networking sites such as Myspace, blogs, Flickr, wikipedia. Its just that people have now begun to associate certain technologies (such as ajax) and graphical styles to web 2.0 even though when Tim O'Reilly first came up with web 2.0 he wasnt including technologies or certain graphical styles.
Well some people have been mentioning web 3.0 and this is where the user will have a permanent connection to the internet and collective intelligence.
But anyway look up Tim O'Reilly's speach to find out more about web 2.0.