Voyeurism is a practice in which an individual derives sexual pleasure from observing other people. Such people may be engaged in sexual acts, or be nude or in underwear, or dressed in whatever other way the "voyeur" finds appealing. The word derives from French verb voir (to see) with the -eur suffix that translates as -er in English. A literal translation would then be “seer” or "observer", with pejorative connotations.
Also, the word voyeur can define someone who receives enjoyment from witnessing other people's suffering or misfortune; see schadenfreude.