Self explanatory. I've read that they stand for rock, paper and scissors, but I can't find any ultimate origin of why these terms are what they are. Thanks!
They are derived from the onomatopoeia used to describe the motions the hand makes when playing.
グー comes from ぐっと, the way you clench your fist
パー comes from ぱっと, the way you spread your hand
According to Japanese Wikipedia: