Oral sex is when one partner uses the tongue, mouth or throat to excite the other partner's sex organs. For males it can include placing the penis in one's mouth and for females licking the vaginal area.
Sometimes oral sex is performed as a way of getting ready for intercourse, but it may be done as an alternative to intercourse.
One can get sexually transmitted diseases from oral sex, such as herpes simplex (which can be passed between mouth and groin or the other way). Some STDs, like HIV, are much harder to pass through oral sex, but it is still possible.
The technical term for oral sex is fellatio if performed on a male and cunnilingus if performed on a female. Slang for oral sex is common in Western cultures, for example: "going down on" or "giving a blowjob" or "giving head."