What's Web2.0?

I came back this blog.
While I was away this blogs, the word “Weblog” became big word. Anybody who uses Google or net service knows this word.
Why?
Recently, I see lot of time the word “Web2.0” on Computer Magazine or Tech-News.
But What is Web2.0? Does Anybody knows the real and concreate view of Web2.0.
I don’t like any more Trend Key or hipe word from those of IT Consultants.