The evolution to Web 2.0, for lack of a better term is about attitude and expectation. Whether it is technology that led to a change of attitude, or that a shift in our relation to the web led to new technology is an academic debate which I will leave to the more technically endowed.
In the 90s, the web was driven by companies seeking to turn it into a giant shopping mall. Consumers are now reclaiming the web for what it was intended for: a collective space bringing people together so that they could share experience and information. Just picture this: a collection of mega websites competing to attract eyeballs v. loose networks accessible by search engines, tags and connections where you can share information, engage in conversations and co-create. I am caricaturing here but the change is quite noticeable...
This is how I understand it: Web 2.0. is a different way of looking at the web.