In the past, sadomasochistic activities and fantasies were regarded by most psychiatrists as pathological, but have been regarded as increasingly acceptable since at least the 1990s. Indeed, the DSM-IV asserts that "The fantasies, sexual urges, or behaviors" must "cause clinically significant distress or impairment in social, occupational, or other important areas of functioning" in order for sexual sadism or masochism to be considered a disorder. Psychiatrists are now moving towards regarding sadism and masochism not as disorders in and of themselves, but only as disorders when associated with other problems such as a personality disorder. People who practice BDSM, as well as most psychiatrists, don't view these practices as disordered.

On a psychological level, much BDSM play involves power and dominance, in particular power exchange of various forms. One person may willingly and consciously hand over personal autonomy or the power dynamic may arise between the parties in the relationship as a spontaneous result of their interpersonal chemistry, in which case no conscious decision is made. This power can manifest in an endless variety of relationship dynamics. Some of the variations include:

Addressing another person as "Master" or "Mistress" for a ten-minute scene.
A witnessed, formal collaring with a lifelong agreement between the parties for the dominant to provide an ongoing presence in the submissive's life. See Total Power Exchange. Collared slaves have responsibilities and duties that vary from the moderate to extreme micro-management.
An agreement of service that covers the duties and responsibilities of the submissive that may or may not include a long-term commitment. Some service relationships are understood to last only as long as the submissive maintains performance standards.
According to most practitioners, the power exchange should always be negotiated. Before play, the participants discuss their physical and psychological limitations, establish safewords (words that will signal the cessation of the scene), and work out what activities they will engage in.

However, many reject extensive negotiation and eschew the use of safewords, preferring instead to accept heightened risk and facilitate a more "natural" interaction. The conflict between the need for risk and the need for limitations and safety is at the heart of the SSC and RACK controversies.

A dominant person enjoys being in control of a submissive person. Reasons for this are said to include demonstrating skill and power, having ownership of another person, and being the object of affection and devotion. Domination may be the fashion in which the dominant feels most comfortable expressing and/or receiving affection. Service-oriented dominants would add that it is obviously useful to have the resources and abilities of another human at their disposal.

Of course, other known possible motives remain to be considered, including pleasure taken not only in sheer power, but in the suffering of others, thrill seeking in risk taking, and outright self destructiveness. That is why many in the BDSM community are concerned with establishing the motivations of those involved in an encounter and advise caution in making BDSM connections.

A submissive person is one who submits of their own free will and seeks to submit to another. Submissives vary in how seriously they take their position, training, and situation. Motivations for engaging in submissive behavior may include relief from responsibility, being the object of attention and affection, gaining a sense of security, showing off endurance, and working through issues of shame. Others simply enjoy a "natural" feeling when they are in the presence of their partner. What are known as service-oriented submissive may also have a deep seated desire to be "of use". Submissives also vary in the extent to which they engage in play, in how often they play, and even in whether they consider their role "play" at all.

In BDSM, a top is a partner who takes the role of giver in such acts as bondage, flogging, humiliation, or servitude. The top performs acts such as these upon the bottom, who is the person receiving for the duration of a scene. Although it is easy to assume that a top is dominant and a bottom is submissive, it is not neccessarily so.

The top is sometimes the partner who is following instructions, i.e., he tops when, and in the manner, requested by the bottom. A person who applies sensation or control to a bottom, but does so to the bottom's explicit instruction is a service top. Contrast the service top with the pure dominant, who might give orders to a submissive, or otherwise employ physical or psychological techniques of control, but might instruct the submissive to perform the act on him or her.

The same goes for bottoms and submissives. At one end of the continuum is a submissive who enjoys taking orders from a dominant but does not receive any physical stimulation. At the other is a bottom who enjoys the intense physical and psychological stimulation but does not submit to the person delivering them. It should be noted that the bottom is most often the partner who is giving instructions—the top typically tops when, and in the manner, requested by the bottom.

Within a sadomasochisic context, submissive is often considered synonymous with bottom. Others opine that a "submissive" is specifically pursuing a dominant/submissive power-exchange as a key element, whereas a "bottom" may or may not be interested (or even willing) to engage in that exchange. For the latter, some have proposed the "pitcher" and "catcher" (borrowed from baseball terminology) as more neutral terminology, with the "pitcher" delivering the sensation, the instruction, etc; and the "catcher" receiving what is "pitched."

TBDSM may or may not involve sex of any kind.
BDSM may or may not involve sexual roleplaying.
BDSM may or may not involve ageplay.
How dominant or submissive a person may be in their regular life does not always determine their preferred role in BDSM play though many people do manifest these tendencies. Often people who express one role in their regular life, such as at work, strongly desire to express the opposite role within their sexual life, as a kind of release.
BDSM play often includes the psychological pleasure of fetishes.
Some BDSM players are polyamorous or are sexually monogamous but engage in non-sexual play with others.
A couple may engage in BDSM sexuality within an otherwise non-D/S relationship dynamic.
When there is abuse in the relationship, the submissive is as likely to be abusive as the dominant.