Deep throating is one variation of oral sex, where the cock is completely inside the throat. The point of this penetration is to get as deep as possible.
Unlike a normal blow job where the tightness his cock feels is not as intense as by other means of stimulation. The difference with deep throating is that the muscles of the neck are tighter and feel more like a vagina or anal sex. One rather vulgar expression (used mostly in the porn industry) for deep throating is face fucking. Both straight and gay couples practice deep throating and they all seem quite contented.
An important tip for the person giving: practice your gag reflex. There are different breathing techniques one can practice (and of course practice makes better) to help reduce or kill off the gag reflex. Also use spit, and lots of it.
Wanna read more about sexy things to do with your mouth and tongue?
Oral Sex, Blow Job, Licking