A person who weirdly watches and observes other people he shouldn't have that much interest in. This could be by way of peering through windows, around corners, or through binoculars.
Also can be applied as a derogatory term to anyone who appears weird or strange.
Guy 1: Dude, Sam's Dad was totally being a creeper again at practice today!
Guy 2: I know, I saw him sitting there in a lawn chair and staring at us with binoculars!