Self-gratification. The act of "playing" with your genitals, for sexual excitement/pleasure; usually to orgasm.
Can also be accompanied by watching/reading/listening to arousing material e.g Porn, films, novels...
No matter what your gender/age/race, people masturbate. Some people do it everyday and more, some people do it when they don't have a sexual partner (these people are in the minority).
Female: Either stroking/rubbing, caressing (vigorously or not) the clitoris, or using long, phallic like objects to insert inside the vagina, and rubbing it back and forth.
Male: Stroking, rubbing, caressing, apparently also 'slapping' the erect penis.
I started masturbating since when 12 (yeah, I'm sure you wanted to know that) since then I have masturbated at least once a day, I am female.