To be voyeuristic is to get excited or interested by watching others. Although this word often has a sexual connotation, it can also be used to for any kind of secondhand excitement.
A voyeur is 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."