It was only after the web 2.0 was named 2.0; the old traditional static web sites were named web 1.0; which incorporates html tables, guestbooks, marquees, gif buttons, etc - the static web.
Some people confuse that AJAX is web 2.0; rather AJAX is just a part of web 2.0, used to build rich/interactive user interfaces. Several other technologies that goes in web 2.0 are like RSS Aggregation, REST, SOAP, web widgets, social networks, taggings, new open architectures, bloggings, wikis, collaboration, sharing;
While web 3.0 is about small web applets (weblets?) peiced toghether on standard interfaces to enrich online user experiences from anywhere any device (web, mobile, pc, desktop, netbook). They say its more about semantic web. Which is more like linked-data(google wave), turning every data into information. Technologies like XML, RDF, OWL, SPARQL.
Semantic Web technologies.
And for web 4.0 someone wrote "
The web become sentient, rises up, conquers the world and enslaves the human race?" (0: