To me, spanking is where someone is hitting someone else's bottom! BDSM is a wide range of bondage, discipline, submission, dominance, sadism, masochism and probably some other things.

I think it's really fluid though. In my travels through the internet I find spanking sites that have things I don't like and BDSM sites that have stuff that really turn me on. But I would say that I'm more into spanking and submission than I'm into bdsm if I had to slap a label on myself.