This topic requires some definitions. These may not be perfect definitions, they may not be your definition, but when I speak or write about these topics, these are my definitions.
Sadism: deriving pleasure, often but not always sexual, from causing pain
Sadist: a person who, in some situations, often sexual, enjoys causing pain
Masochism: deriving pleasure, often but not always sexual, from feeling pain
Masochist: a person who, in some situations, often sexual, enjoys feeling pain
Sadomasochism: the umbrella term that refers to both sadism and masochism
Sadomasochist: a person who is both a sadist and a masochist
S&M: the abbreviation for sadomasochism.
S&M can be either a good thing or a bad thing. If someone is sadistic without concern for the person on whom they inflicting pain then they are likely a sociopath or a psychopath. Masochism, in the form of seeking pain or inflicting pain on oneself as an emotional crutch, as an addiction, to punish oneself, to escape a negative life situation, or for other similar reasons can be a symptom of a mental illness. Sadism and masochism, especially for sexual gratification, are things that many people play with consensually in relationships.
Fetish: a form of sexual desire in which gratification is strongly linked to a particular object, activity or a part of the body that is not generally seen as sexual
Kink: a non-conventional sexual practice
Fetishes generally have more to do with attraction, and in many cases are required for sexual gratification, but colloquially, the terms have become interchangeable.