BDSM is broken into three subcategories: Bondage/Discipline, Dominance/Submission, and Sadism/Masochism.
In the most well-known S & M aspect, S meets his/her desire to control and abuse through physical domination of M while M meets his/her desire to be sexually controlled and abused through physical doination by S.*
*S is not only referred to as Sadism, but also some people say S = Slave; while M is not only referred to as masochism, it is also said that M = Master