Form of sexual role playing, where one person enforces rules through the use of pain and humiliation techniques.
In a relationship, in which one is the dominant, who attempts to control the actions of another, by the use of rules, and to enforce those rules, uses various methods of punishment. True Disciplinarians include 'training' of the submissive, to the rules, and the penalties for violating those 'rules'.
A great deal depends on the 'rules' between the participating parties, and exactly what will constitute punishment for breaking of those rules.
Discipline in the BDSM context, is about the pleasure derived by one person, who manages to control the actions of another person, rather than due to the infliction of pain, or humiliation.
Though both pain, humiliation, are part of any 'discipline routine' it is dependent on the one violating some code of conduct, some set out rules.
For the most part, the rules are stated beforehand, along with the associated punishment for breaking them.
This can include bondage, in which one is restricted in their movement, but rather than for the pleasure of being bond, or having one incapable of movement, it is due to the violation of rules, as a form of punishment.
Practice (Associated Acts): Rules are the determined guidelines for a submissive's behavior, and are usually clearly defined.
Some will make up the rules on the fly, in a 'training' session, or in a more SandM scenario than in a true Discipline session.
Punishment is the determined result for violating the rules, and can be painful or humiliating or both. It can be mild from simply restricting a submissive from having an orgasm for a few days or longer, to caning or other methods.
The difference between this and say SandM (sado masochism) is that the pain is training method, a consequence of one's action, rather than purely for the pleasure derived from the giving and/or receiving of pain, humiliation.
Discipline, itself, is more about control, than anything else.
It is also used in Slave / Master scenarios, as a further means of controlling the behavior of the slave and in establishing who is the owner, who is the property.
Noteworthy: Humiliation can take many forms, such as physical methods like using an enema to make a person defecate, to verbal abuse. It depends on the parties involved.
This also holds true with pain as being a punishment for rule violations.
It should be noted, that even with the acts being consented to, prior to happening, is not necessarily a legal defense in some jurisdictions.
Training refers generally to acts or a series of acts, to which one who is dominant, the master, shows the submissive, the slave, what is required of them, in order to be a proper slave.
This includes discipline as it applies as one being in control.
Random application of rules can be done to keep the submissive off balance, again re-enforcing one's absolute control over the other.
In more SandM (sado masochism) leaning, this is done more to create an opportunity to inflict pain or humiliation techniques.