What does cinematography mean, anyway?

I'm not madly trying to see all the Oscar-nominated movies in the next couple of weeks, but I have
been looking through the nominations. I realized that I'd seen 4/5 of the films nominated for Cinematography, so I rented "Black Dahlia" in order to complete the set. It's not a good movie, but it definitely is atmospheric, and since I was looking specifically, I guess I can see the reason for its nomination. I started wondering what cinematography means, really. I tend to think of sweeping vistas and long shots, but the dictionary definition is pretty simple:
"the art or science of motion-picture photography." Not helpful. Pauline Kael, writing dismissively of "Dances With Wolves" (which won for cinematography in 1990), called it "Plays with Camera."
This year's nominees
http://www.oscar.com/nominees/?pn=detail&nominee=ChildrenOfMenCinematographyNominee
might mostly be called "plays with light" but that doesn't explain "Children of Men." I liked it a lot, and thought it should have been nominated for best picture, but I don't know that its
photography was so distinctive.