A person who submits control of a large percentage of his or her day-to-day life to a dominant partner, or who submits within a formal set of rules and rituals, is the slave, and the person who assumes power over the slave is the master or mistress.
Agreement on the exact meanings of the following terms is far from universal. For example, the function of tops and dominants is similar, and, in many cases, overlaps, but while the terms are used interchangeably in some discussions, there are differences between the two.
The top is the actor within the BDSM context who applies to the bottom partner(s) the techniques of a sexual, sensual, and psychological activity: flogging, bondage, servitude, or humiliation. The dominant controls the BDSM scene or relationship, exercising authority over the submissive partner(s). A dominant who exercises control over a large percentage of a submissive's day-to-day life, or within a formal framework of rules and rituals, is a master or mistress.
The functions of top and dominant often intersect, where the top is the dominant, but this is not always the case. Someone who is "topping" may be doing so at the request, or even the direction, of a bottom partner; in this case, the bottom is the dominant partner. A top who acts within this kind of relationship dynamic is sometimes called a service top. A bottom who has dominance over the activities or the relationship is said to be topping from the bottom, even though he or she is really exercising dominance from the bottom. Another possibility is that the top and bottom are acting at the direction of a third, directing person.
Within communities of lifestyle BDSM devotees, there exists a widespread prejudice against both those who act as service tops and those who top from the bottom. Both are considered by many to be failing to achieve a proper BDSM relationship dynamic, especially, if the partners are purported to be trying to achieve a dominant-top/submissive-bottom relationship.
While it is possible that a dominant would not act as a top and thus have no expression of his or her control through kink- or fetish-based activities, it may be argued that such a relationship, lacking any erotic aspect to the exercise of control, would fall outside of the BDSM context.
Tops or dominants who also assume a bottom or submissive role are referred to as switches.
The only prevalence data on roles currently available is anecdotal and not statistically significant.
Master or Mistress 
Master or "Mistress" is one of the honorifics some people use to describe the dominant partner or "owner" in a Master/slave relationship. Because it is an honorific of the dominant form it is usual for it to be written with a capital letter.
It might also be used by the submissive partner as an honorific term in a D/s relationship. This can cause confusion when trying to understand BDSM terminology as both "Master" or "Mistress" and "slave" might be used - as terms of endearment - even though neither considers himself to be owned nor owner.
Some people enjoy the connotations of servitude or submission in calling their partner "Master", or being called "Master". To others the ritual of such a formal mode of address may be appealing. It may also be useful in building roles in which one partner or the other may indulge in sadistic or masochistic desires, although people living as Master and slave are not necessarily sadomasochistic.
Usage of "Master" or "Mistress" in most BDSM environments does not imply any specific expertise, abilities, or formal training. To successfully maintain a Master or Mistress/slave relationship takes abilities and skills beyond or apart from normal relationship skills.
The term "Master" can be gender-independent but is mostly used only in reference to males. There is otherwise no male-specific equivalent. The female equivalent is "Mistress" or possibly dominatrix.
Although the Master is understood to have authority over the slave in some sense, this never extends to one's legal rights and thus there must always be an implicit element of consent involved.
Power and Safewords 
The control of the dominant over a partner is seldom absolute and often operates within a set of defined limits.
One such limit is a safeword, a signal that a submissive uses to communicate with his or her dominant about the scene while it's in progress. Safewords can communicate that a limit is being neared (i.e. "yellow") or that a limit has been reached (i.e. "red"). Safewords are often negotiated before the scene (although saying "safeword" is usually recognized as a safeword). In some cases, safewords are used to completely stop the scene. Accepting more risk, a submissive may agree to forgo a safeword, consenting to edgeplay, an extreme form of submission.
See also 
References and further reading 
- Dossie Easton, Janet W. Hardy. The New Topping Book. Greenery Press, 2003. ISBN 1-890159-36-0.
- Jay Wiseman: SM 101: A Realistic Introduction. Greenery Press (CA) 1998, ISBN 0-9639763-8-9
- Saez, Fernando y Viñuales, Olga, Armarios de Cuero, Edit. Bellaterra, 2007. ISBN 84-7290-345-6
- Phillip Miller, Molly Devon, William A. Granzig (Vorwort): Screw the Roses, Send Me the Thorns: The Romance and Sexual Sorcery of Sadomasochism. Mystic Rose Books 1995, ISBN 0-9645960-0-8
- William A. Henkin, Sybil Holiday, Consensual Sadomasochism : How to Talk About It and How to Do It Safely, Daedalus Publishing, 1996. ISBN 1-881943-12-7.
- Breslow, Norman: SM Research Report, v1.1, 1999
- Janus, Samuel S. / Janus, Cynthia L., 1993 The Janus Report on Sexual Behavior, Wiley, New York
- Thomas S. Weinberg: S&M – Studies in Dominance and Submission (Ed.), Prometheus Books, New York, 1995 ISBN 0-87975-978-X
- Robert Bienvenu, The Development of Sadomasochism as a Cultural Style in the Twentieth-Century United States, 2003, Online PDF under Sadomasochism as a Cultural Style
- Charles Moser, in Journal of Social Work and Human Sexuality 1988, (7;1, P.43-56)
- Gloria G. Brame, BDSM/Fetish Sex:Overview and Study, online gloria-brame.com