What makes Twitter different than Facebook?

[Football fan]

From what I know, You tweet what you want to say and your "followers" will know what you're doing or thinking of. But is there more to it? Because if that's about it, then it sounds lame and I don't see how it caught on. The only reason I bothered asking this is because I see it has caught on and I hope I won't see a day when Twitter has more members than Facebook if what I said was true. Sorry for my bias, I guess I just like having only one social network account.