A voyeur is someone who gets excited by observing an unexpected stranger who is undressing, naked or engaging in sexual activity.
Looking and being looked at is an important part of anyone’s sexuality. But normally this involves a consenting adult. You only speak of voyeurism if it is someone who looks at someone without the other knowing or wanting it. The risk of being caught is part of the thrill.
Although there are female voyeurs, most of them are guys. A male voyeur is also called a ‘peeping Tom.’
For some people voyeurism is their only sexual outlet. But most voyeurs have normal sexual relationships. They only satisfy their voyeurism once in a while, looking for unshaded windows through which to glimpse at someone disrobing or at a couple having intercourse.

See also: exhibitionism