An umbrella term for a great number of different activities, BDSM is an initialism consisting of a few different parts (according to its first known usage on UseNet alt.sex.bondage in 1990), where it has now settled to the following definition:
Bondage and Discipline (see Bondage)
Dominance and Submission
Sadomasochism and Masochism (see Sadist and Masochist)
Most typically, a BDSM scene will involve a Submissive and Dominant, and a well-thought out scene will usually involve some form of Aftercare.