1, 2, and 3 Dimensional Characters: How Do You Tell Them Apart

This is something I'm still confused about. I've read lots of books and seen lots of movies, but I have yet to determine which characters in a story are one dimensional, two dimensional, or three dimensional. Frankly, I don't know the difference between them, except that three dimensional characters are more important and appear much more often in the story. But that's just about it to me.

Lots of movies these days, especially those on made for tv are criticized for having a cast of mainly two dimensional characters.

So can anyone help me with this issue? Exactly how can you determine which character are which?