A good introductory video to watch is: Web 2.0..The Machine is Us/ing us:
[youtube=http://www.youtube.com/watch?v=6gmP4nk0EOE]

and an interesting powerpoint:
[slideshare id=370508&doc=web3evolution-1228454469401752-9&w=425]

and last but not the least, Web 3.0 according to Eric Schmidt (Google):
[youtube=http://www.youtube.com/watch?v=T0QJmmdw3b0]