For quite a long time, s/m was considered to be a psychological disorder and treated as a mental illness. Through the breakthroughs we have made in the field of psychology, it has finally been learned that s/m is not a mental illness, nor does participation and/or enjoyment of s/m activities predispose a person to being a criminal. 

S/m, simply put, is the enjoyment of giving or receiving pain. An s/m relationship is comprised of a sadist and a masochist. The sadist gets pleasure out of giving pain. A masochist receives pleasure out of receiving the pain. Within BDSM, s/m includes mild pain play, to intense edge play. However, it is not a necessity to include BDSM in a d/s (dominant/submission, power exchange outside of sessions or sex) relationship. 

An s/m relationship is one that contains a sadist and a masochist who engage in s/m play activities, without a power exchange being present outside of those set play times. In this instance the sadist would be a Top and the masochist would be a bottom. However, this is not necessarily true in all cases. A sadist and a masochist can get together strictly for play in a play partner relationship. It also is not necessary for a sadist to be a dominant, or a dominant to be sadist. 

The main focus of a strictly s/m relationship is on physical sensation. These sensations are often intermixed with sexual pleasure and sexual acts. S/m does not mean that being slapped or stubbing one's toe is pleasurable. For many there has to be a mood involved as well. Often s/m activities are a prelude to sexual intercourse, though there are time where the s/m activities do not lead to sex. At those times, the s/m play is purely for the physical and mental pleasure they give. Like any BDSM interaction, s/m play and relationships are consensual in nature. Without consent it would not be a real s/m interaction or relationship. it would be abuse.