Dominance and Submission describes a relationship in which there is a dominant person and a submissive person - one has control over the other in some way or aspect of life, it could be only within the bedroom or it could be in all areas of life, or it could be a casual encounter.
There are many different styles of D/s relationships and there is no right way or wrong way, it is like any other relationship – what works for one set of people may not work for another.