I honestly do not really consider this cheating, but I was in a relationship with a man who was cheating on his wife. He and I had more or less ended things, but we still had all these messy feelings, but I slept with his best friend. Mostly to get him to back off and leave me alone, but it was still wrong and mean of me. And I do feel bad about it, but I don't really consider it cheating. I just consider it mean.
Otherwise, I have never cheated, but I'm pretty sure that I have been cheated on. I never knew about it then, or else I'm positive I'd be pissed and angry and hurt. After the fact, I don't really care.