One of the main ways of achieving orgasm is by direct or indirect stimulation of the penis or clitoris. Such stimulation can come from a variety of activities, including but not limited to sexual intercourse, manual masturbation, anal sex, oral sex, non-penetrative sex, a sensual vibrator, or an erotic electrostimulation. Orgasm may also be achieved by stimulation of the nipples or other erogenous zones. In the absence of physical stimulation, orgasm can be from psychological arousal alone, or during dreaming (a nocturnal emission or "wet dream" in males).
Important in sexual stimulation are internal glands, called the Skene's glands in women and the prostate in men, two homologous structures. In common use, the term G-spot refers to these areas.
The stimulation can come from receptive intercourse, fingering, fisting, or penetration with a dildo.
With sufficient stimulation, the prostatic structure can also be "milked." Providing that there is no simultaneous stimulation of the penis, prostate milking can cause ejaculation without orgasm. When combined with penile stimulation, some men report that prostate stimulation increases the volume of their ejaculation.
--http://en.wikipedia.org