so theres facebook/twitter, and blogs, where does tumblr fit?
I consider it like a mix between twitter and a regular blog site like word press. It's really easy to use, very simplified. There are all kinds of things on it. It's easier for finding photos and viewing photos than twitter is. It's got everything from hard core porn to lolcats.
I really like history so I subscribe to a lot of people who post photos from WW2 or things form the medieval era.
Basically it can be whatever you want it to be. Just check it out, search a topic you are interested and see what comes up.