BDSM means different things to different people. Technically, it is an amalgam of several different practices: bondage/discipline, Domination/submission, and sadism/masochism. But it is so much more than that. It can range from the very tame to the downright deadly (I obviously don't condone the latter...I love BDSM, but it's not worth dying for). For some people, it's a way to spice things up a little in the bedroom; for others, it's a lifestyle. There is a long, long list of activities that fall under the BDSM umbrella, and I'm not even going to attempt to list them all here. Some examples include: anal play, corporal punishment, bondage, fisting, mummification/sensory deprivation, total power exchange, sissification/forced feminization, role playing, age play, CBT (cock & ball torture), NT (nipple torture), medical play, enema play, queening/face-sitting, foot fetish/body worship, etc. I haven't even scratched the surface!
If you are a novice, I suggest you read as much information as you can. The internet has some great sites devoted to BDSM; as well, many good books have been written about the subject. When it comes to BDSM, the more you know, the better.
And always remember the BDSM mantra: safe, sane, consensual.