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Domination and Submission (Dom and Sub) is basically two people (can be more, but is very commonly 2), one is dominant and tells the other person what to do, the other person being the submissive who takes the orders and does what the Dom says. In most cases the Dom and Sub have a healthy and loving relationship between them and are taking bondage and discipline to a higher level.
Sadism and Masochism is all about either receiving pain (Masochism) or inflicting pain (Sadism) to another person and in both cases the person gets sexual satisfaction from doing it or receiving it. This can tie in with the other elements of BDSM, like bondage for example.
So those of you who are wondering what BDSM is ill tell you what it is now. Its Bondage and Discipline (BD), Dominant and Submissive (DS), AND Sadism and Masochism (SM). The more you know.