To be voyeuristic is to be sexually excited by watching others. Peeping Toms are voyeuristic.
A voyeur is a word for a Peeping Tom, someone who likes to watch people who are engaging in private activities. Anything voyeuristic therefore has to do with watching or spying on someone. Voyeuristic activity usually creeps out the people being watched, because it violates their privacy. The word voyeur is French, meaning "one who views," from the verb voir, "to see."