BDSM is a term most often used to describe a range of sexual behaviors, but it's also an approach to sex and sex play. The term comes from three other acronyms:
B&D, which stands for bondage and discipline
D/s, which stands for dominance and submission
S&M, which stands for sadism and masochism
BDSM isn’t a clinical term universally used by medical or health care professionals; it’s more often a term used by people to describe their own sexual practices, and sometimes used by others to denounce the way others choose to have sex.
There isn’t one accepted definition for BDSM. A very general one might be that BDSM is a form of sexual expression that involves the willing and consensual exchange of power. Notice that the definition is “sexual expression” and not just sexual activities. A lot of people who engage in BDSM talk about the fact that most of it happens in your mind, and often the sexual activities you can see are the least interesting aspect of the action.
An example of BDSM might be using a pair of stockings to tie your partner to a bed post during sex to elaborate scenarios of dominance and submission that include role play, costumes, and days of preparation (and often no intercourse at all).
The key to understanding BDSM is that it only describes actions that all parties choose to participate in. Any sexual activity that is unwanted or forced upon someone would not be considered BDSM -- it would be considered harassment or assault.