Master-Slave Relations
In BDSM, Master/slave or M/s is a relationship in which one individual (the submissive) gives to another (the dominant) ultimate authority over them. It is a form of dominance and submission. The participants may be of any gender or sexual orientation. The relationship is structured in terms of slavery, because of the association of the term with ownership of the slave and the rights of a master to their body, as property or chattel. The dominant is often called Master if male, or Mistress if female.
Who's in control depends on the severity of the D/s relationship. Some folks have rather mundane, ordinary relationships, involving every aspect of their lives, from taking care of their kids, to the kinky stuff they do in the bedroom. In that case, there is generally a consensus regarding WHEN and HOW to get into power exchange situations. Then of course, there's an agreement as to how "deep" that power exchange goes. Sometimes the "slave" who in this case, is more likely referred to as a "submissive" has a great deal of control. Sometimes she has none. It depends on their arrangement and ordinarily recognizes that people (even submissives) get INTO relationships because of what they get OUT of them.
There are also very strict master/slave arrangements in which the slave has minimal power. S/he decides to get into the situation in the first place, but after that she considers herself owned and only has contact with the master/mistress for their pleasure and NOT the pleasure of the slave. There are people who are very hard core about this concept, insisting that the slave, once stripped of control, has ZERO choices.
Most BDSM relationships fall somewhere in between.