Will someone please explain to me what a "compact crank" is and why they're so popular?

I know what "compact" means, and I know what a "crank" is--yes, on the bike as well as in other areas of life . But put them together and all I can do is display my ignorance. Please enlighten me. Thanks!

Because I know you can't all be talking about some short guy who's hard to get along with.