• Tim O'Reilley the so-called creator of the term... (50'')

The article written by Tim O'Reilly in 2005 is still a reference to the concept of the term. The article can be found here. In fact, there is also an audio version of the article, that can be found here. Another great article comparing web 2.0 and 1.0 can be found here.

• A REALLY cool definition without words (4'30'')
  

and then the response to the COOL video, which is worth watching (3'16''):

• And a not as cool but accurate definition with many words (5'18''):

• Here is the definition given by Google's CEO (1'50''):