I saw this question asked on a forum a while back and thought it was quite interesting.
At first the two terms seem almost synonymous, one being a more technical (medical?) way while the other is much more in the common vernacular. But the more I thought about it, the more I realized that it was highly possible for someone to be a masochist, and not be a painslut.
According to the DSM-IV sexual masochism can be defined as: