You can read through these articles to get a hint..
http://www.oreillynet.com/pub/a/orei...is-web-20.html http://en.wikipedia.org/wiki/Web_2.0
But basically Web 2.0 sites are sites that offers more interaction with the visitors/users compared to the "classic" web 1.0 sites that presents only information.
AJAX is often used to create sites that looks more like applications rather than traditional sites.