From SC2Pod
A detector is a unit or building that has the passive ability Detection and thus can see invisible units within its sight range. These passive detectors are the Missile Turret, Photon Cannon, Spore Crawler, Raven, Overseer and Observer.
Detectors reveal the units to all allies.
Some units and buildings have abilities which - among other effects - also reveal cloaked units. Examples are the Scanner Sweep of the Orbital Command, the EMP Round of the Ghost and Fungal Growth of the Infestor.
