What is MobileMe and how much does it cost? What can you do with it...?

I've heard of MobileMe but I don't know anything about it, except that if you have an iPhone 3G S, you can trace it with MobileMe if you lose it. That's why I want to know more about it. What is it? What else can you do with it? What does it cost? What do I need to know about it? Please tell me as much as possible about it. Thanks!|||http://www.apple.com/mobileme/