Web 2.0

Does anyone know of a good definition of web 2.0 somewhere? It seems like one of those vague things…
Most of the sites that talk about it engage in far too much pretentious wankery for me to accept their definitions
The concepts are “users contribute content” and “content from different sites can be merged”. Also, “gradient backgrounds make us look cool” “Javascript is neeto” and “Kittens! OMGLOL!”