In "cell division", what do G1, S, and G2 stand for/mean?

In Bio cl our teacher gives us a sheet at the beginning of a unit with all of the vocabulary and concepts we need to know, and G1, S, and G2 are on it, but i don't think he has told us what those mean yet. I think they have to do with Mitosis and Meiosis, but I'm not sure.