Wikipedia describes oral sex as:

Oral sex is sexual activity involving the stimulation of the genitalia of a sex partner by the use of the mouth, tongue, teeth or throat. Cunnilingus refers to oral sex performed onfemales while fellatio and irrumatio refer to oral sex performed on males. Analingus refers to oral stimulation of a person’s anus. Oral stimulation of other parts of the body (as in kissing and licking) is usually not considered oral sex.

People may engage in oral sex as part of foreplay before sexual intercourse, or during or following intercourse. It may also be performed for its own sake.