The succubi are typically female spirits (incubi being the male counterpart) that feed of sexual energy.
However, as Personified said, some people tend to lump them into the same category due to the actions of a few. Like us, they can be pleasant, unpleasant or anywhere in between- not all are particularly malevolent.
Typically, it is said a succubus can either be in dreams or not. Some have said that they have experienced intercourse with a succubus in the Astral.
A succubus is also said to collect semen from the men she has fornicated with. It has often been said that the succubus are not capable of reproduction, however there is controversy about that statement. It has been suggested that the incubi use semen to impregnate human females.
If you find the succubus interesting, you may also want to delve into the following: Al Basti, Empusa, Lamia, Lilith, The Lilin, Mara, Rusalka, and Belili. These are similar figures in mythology from a variety of cultures.
Here is a site you may find interesting, although I can not guarantee its validity.
That site also has more information in tabs on the left hand side of their page.
As also said previously, if you type in 'Succubus' or 'Succubi' in the search bar either on here or Google, I am sure you may find some information on them worth knowing.