A Spectator is a player who has decided not to take part in the game and just watch, or has lost all of their lives (Zombie, Survival Mode). They can freely travel through the map in noclip, but cannot interact with the map (using switches, breaking barricades etc. They can, however, still contact players through voice chat or through text. A "cheating" technique used by some teams is to have a spectator keep tabs on who is where and what they're doing (e.g. telling a group of Survivors who are inside a room with many entrances where the Zombies are coming from, or where they can go next.
Becoming a Spectator
Most maps have a trigger zone as with Human or Zombie teams, but some lack the trigger zone for spectators. This is fixed with using the keyboard shortcut (default F3) to enter spectator.