Web 2.0 – help!

For a while now, I have read various posts in my rss-feed reader all mentioning Web 2.0.

But I haven’t been able to find what this is exactly.

Some sites mention AJAX, which I only know as a Greek hero and a football club.

Help?

Can someone give a clear example/explanation on Web 2.0?
What is it?
Help!

[link found on Tim O’Reilly website: What Is Web 2.0 – Design Patterns and Business Models for the Next Generation of Software]