What is Web 2.0?

I've seen the term "Web 2.0" a lot around the web over the last couple of months. I don't know what Web 2.0 means. I know versions of software programs change as upgrades are created, but it doesn't seem like that is what this is (how can the entire Internet be "upgraded" like Vista - still out on the success of that upgrade BTW)? It doesn't make sense to me, but everybody is talking about it. What the heck is it? Anybody want to enlighten me please?