Web 1.0 sites are static, monolithic, and unidirectional. Think Drudge.com.

There is no exact consensus of what is (or isn't) Web 2.0, but a new generation of websites are embracing the same set of new techniques and new behaviors, and these are generally recognized as Web 2.0.

Web 2.0 sites are dynamic (rather than static).
They are interactive; users can elicit information without loading a new page.
They are participative; the audience makes comments and contributions.
They are multidirectional. (push/pull) (Google alerts)
User participation adds value to the application / website. (network effects) (Ebay ratings)
They are social networks (FaceBook, LinkedIn) and online communities (Second Life).
The audience contributes content and recognizes authority.