What is "other"?

I'm trying to update my MBP to the latest version of Lion, but my startup disk is so full it says I can't do it. The problem is, when I look at where all my storage is being used, so I can clean it up a little - I'm confused.

I can't make this image embed, but here's what I'm seeing.

First: I don't know what "other" is. I can't imagine that documents are taking up that much storage.

Second: I've looked through my movies in the "Movies" folder and on iTunes. It's only about 15GB - so where is the other 78GB hiding?

I'm a rookie to this stuff, so please be nice. Has anyone else had this problem?