Observers are small robotic aerial scout drones used by the Protoss. These robots are used for reconnaissance and spotting invisible enemies, and thanks to their remarkably powerful sensors, they excel at their roles. Observers also utilize cloaking fields to compensate for their lack of weapons and relatively weak armor and shields.
Using Observers: The Observer becomes available once you gain Protoss Advanced Tech Tree. After getting a Robotics Facility, some would prefer to get the Observatory next rather than the more expensive Robitics Support Bay, which grants the Protoss Reaver and upgrades for the Protoss Reaver and Shuttle. The Observer has a wide field of vision and is invisible, so it is safe unless if there are Detectors nearby. More importantly, Observers can detect invisible units, which is invaluable against Ghosts, Wraiths, Dark Templar and Arbiters. Keep Observers away from Detectors, or they're good as dead.
Fighting Observers: These little buggers are invisible and can detect any of your invisible units. Counter them with detectors of your own to expose them, then leave the rest to your anti-air units. Using AoE spells like Ensnare or Psionic Storm can reveal them or kill them. Using the Zerg Queen's Parasite will allow you to see whatever the Observer sees, and it can see a lot. If it returns to its home base, you can get a nice sneak peek at your enemy's defenses.
Unit Class: Protoss Robot/Aeirial Scout/Spy
Built by: Robotics Facility + Observatory
Armor: Very Light
Permanent Cloaking: The Observer's main protection is its cloaking field, which remains active until it dies.
Detector: Powerful sensors allow the Observer to detect invisible enemy units.
Plasma Shields: Increases strength of Protoss Plasma Shields. Researched at Forge.
Air Armor: Increases armor strength of Protoss Air Units. Researched at Cybernetics Core.
Gravitic Boosters: Gravitonic thrusters allow the Observer to move faster.
Sensor Array: Improved sensors enhance the Observer's field of vision.