Orgasm denial is often used by a dominant as a form of training or punishment or simply to demonstrate the power they have over the bottom. Sex is still possible under orgasm denial, just the number and timing of orgasm is controlled. (For complete denial see here.)
Short term denial is often used to heighten sexual pleasure. The denied partner is often kept on the plateau before orgasm for some time without permission to orgasm. When (if) eventually permitted to orgasm, the orgasm tends to be much stronger than normal.
Long term denial is denial for an extended period of time, often weeks or even months. This is often combined with the wear of a chastity belt.
Long periods without orgasm can have both mental and emotional effects that are very different between men and women.
Men under denial usually go through several phases. The first several days are characterized by increased aggressiveness most most men. After that, most men start exhibiting signs of depression. These signs include bad mood, muscle pain and irritability and they usually grow stronger as time goes on. For longer periods of time, there is research that suggests that the prostate can take damage because of fluid build up.
Women usually exhibit much weaker symptoms. For most women, after a certain time the desires decreases. There is also no research showing any physical effects on the female body.