Wikipedia
Deep-throating is a sexual act in which a person takes a partner's entire erect penis deep into the mouth and throat. The practice is a type of fellatio when the giving partner is in control, while deep throat penetration by an actively thrusting partner is called irrumatio; both are a form of oral sex. For deep-throating, the penis has to be long enough so that it can reach the back of the recipient's throat. The man whose penis is inserted is usually referred to as the receiving partner, and the person who performs deep-throating is referred to as the giving partner. The technique and term were popularized by the 1972 pornographic film Deep Throat.
It is difficult for most people to perform deep-throating, due to the natural gag reflex evoked when the soft palate is touched. Different people have different sensitivities to the reflex, and with practice, some people learn to suppress it. Deep-throating leads to an entirely different kind of oral stimulation in comparison to regular fellatio, while the giving partner's tongue is immobilized during deep-throating and they are unable to suck; the receiver's glans penis can be intensely stimulated by the tightness of the giving partner's throat.