Definition: BDSM refers to certain patterns of human sexual behaviour. There are many subgroups: Bondage and Discipline (B and D), Domination and Submission (D and S) and sadomasochism also known as sadism or masochism (S and M).
Lots of BDSM performances are carried on neutral or non sexual environment, so they can be perceived as unpleasant, undesirable or outrageous. For instance, while pain, physical limitation and slavery are normally forced against the will of people, but in BDSM these activities are carried out with mutual consent of both the parties, who enjoy this kind of sexual experience.
This focus on consent and security is known as SSC (Safe, Sane and Consentual). Other people prefer RACK (Risk Aware Consensual Kink) because it focuses more in the fact that BDSM activities are potentially dangerous.
There is an argument about the meaning and intention of these words, but both terms refer to the knowledge by the parties and the acceptance of the danger of BDSM.
You can find some examples of this kind of practices in this BDSM videos
