B&D stand for Bondage and Discipline. D&S stand for Domination and Submission. S&M stand for Sadism and Masochism. Crunch it all together and you've got BDSM.
Firstly, let me assure you all that this isn't something mean or abusive. Consenting adults who take part in BDSM like what they're doing. Yes, even the people who are tied up and getting spanked are enjoying it.
I've enjoyed taking charge and I've enjoyed letting someone else take charge. I can tell you from personal experience that both sides of it can be fun.