Succubus

In folklore traced back to medieval legend, a succubus is a female demon or supernatural entity that appears in dreams, who takes the form of a human woman in order to seduce men, usually through sexual intercourse. The male counterpart is the incubus. Religious traditions hold that repeated intercourse with a succubus may result in the deterioration of health or even death.

In modern fictional representations, a succubus may or may not appear in dreams and is often depicted as a highly attractive seductress orenchantress; whereas, in the past, succubi were generally depicted as frightening and demonic.

Source: http://en.wikipedia.org/wiki/Sucubus

Incubus

An incubus (nominal form constructed from the Latin verb, incubo, incubare, or "to lie upon") is a demon in male form who, according to a number of mythological and legendary traditions, lies upon sleepers, especially women, in order to have sexual intercourse with them. Its female counterpart is the succubus. An incubus may pursue sexual relations with a woman in order to father a child, as in the legend of Merlin. Religious tradition holds that repeated intercourse with an incubus or succubus may result in the deterioration of health, or even death.

Source: http://en.wikipedia.org/wiki/Incubus