Voyeur
A song by the band blink 182 where it describes a boy who follows a girl to her house, hides in her tree day and night watching her change.
The word itself describes one who gets sexually aroused by hiding and spying on a couple ad/or 1 person who is doing somthing sexualy arousing.

Guy 1: dude! me and my girlfriend fucked for like 5 hours straight on saturday!

Guy 2: I know, and i'm the voyeur that got it all on camera and put it online this morning!