Sexual activity involving the use of the mouth, tongue, etc., to stimulate the sex organs. Done on the genitals of a man (especially the penis), it is technically called fellatio. Done on the genitals of a woman (the vulva and especially the clitoris), it is technically called cunnilingus. Most people usually use euphemisms or slang terms when talking about oral sex.
Many people enjoy receiving oral sex. Many do not. Many people enjoy performing oral sex. Many do not.
Oral sex has been taboo to a greater or lesser extent in many Western countries since the Middle Ages. Some activities are actually illegal in some jurisdictions.
Oral sex is considered a superb method of contraception (birth control), since pregnancy is impossible unless semen (sperm) comes into contact with the vagina or vulva. Many heterosexual people often use oral sex for this reason.
Many people find oral sex very pleasurable. In particular, receiving oral sex is for many women one of the better ways or reaching orgasm. Thus many people often use oral sex for pleasure even when not concerned about avoiding pregnancy.
Oral sex can be enjoyed in a wide variety of positions. Normally one partner performs while the other receives, but it can be performed by both parteners at the same time in a "sixty-nine" position.
Oral sex is, in and of itself, safe and healthy. Male semen contains water and small amounts of salt, protein, and fructose sugar, and is in itself harmless in the mouth or when swallowed. The female sexual fluids are likewise harmless. Partners should be careful not to bite or scratch with the teeth harder than desired. It is possible for the inside of the mouth to be bruised by the penis if oral sex is too rough. Air should never be blown into the vagina. It is possible for an air bubble to enter the circulatory system in this way and cause embolism and death.
It is possible to catch sexually transmitted diseases, most notably AIDS, from oral sex. Any kind of sexual or other contact with the sexual fluids of a person infected with the HIV (AIDS) virus should be avoided.