If you're watching something, you're a spectator. Football stadiums and circus tents are full of spectators.
The spectator is a particular kind of viewer; unlike a witness or an onlooker, they usually have chosen intentionally to regard the spectacle before them. Usually the word spectator refers to people watching games or "spectator sports," but you could be a spectator at any planned event. An easy way to remember this word is to think of spectacles, glasses used to view something clearly — both from the same Latin root spectare.