Web 2.0 (a bit off topic)

"Microsoft is on its somewhat kamikaze mission to become a force in the Web 2.0 world"

The other day, my girlfriend (that's right - I'm NOT a Linux user :-) ), asked me what "Web 2-0" was. Someone had said something to her on the lines of "bla bla ...facebook ...myspace ....bla bla ...you tube ....you know - Web 2-0".

This got me thinking - what IS "Web 2-0"? When AJAX first came on the scene about (what?) 18 months ago - something like that, I started hearing "Web 2-0" and thought it referred to AJAX and similar techniques. Have I been wrong all this time? Does it REALLY refer to facebook, myspace , you tube, blogs, etc? ie - perfectly ordinary web sites that ...well ....that what?

What IS Web 2-0? Other than just some silly buzz word on the same lines as "extreme programming". Can anyone tell me?