A form of foreplay that can be done by either partner. ex. cunnilingus
1) If a guy does it for a girl, he sucks her clitoris
, or tongue-fucks her..etc. Whatever pleases her, since all women are different. Also done between lesbian lovers.
2) If a girl does it to a guy (or a guy to another guy) she sucks his dick, plays with his balls, etc...whatever gets him off.
Basically, the mouth is used as the main source of pleasure giving.
Most find this as a way to truly express love for their partner because they really aren't getting anything back from it...unless they enjoy the taste.