This link :http://www.kids-in-mind.com/r/road.htm explains it pretty well. Basicly, a lot of voilence and some sex/nudity, and a handful of choice words.
It’s about a post-apocalyptic world in which all surviving humans are cannibals. The main characters are trying to save themselves from being killed and eaten. That’s why.
I don’t know about the movie, but if it’s anything like the book it should be rated “B” for bleah.
I thought it was one of the dumbest (and most depressing) books that I’ve ever read in my life, and I read all the time. (And I read a lot of dumb books.)