The Stalker is a Dark Templar cybernetic war machine armed with carapace-mounted Particle Disruptors adept against both ground and aerial targets.
The fall of Aiur demonstrated to the Dark Templar that more than just stealth and guile were required to defeat the Zerg. Hence, the Dark Templar developed the Stalker, a war machine inspired by the Khalai Dragoon. While Dragoons were piloted by crippled protoss, the stalker is controlled by the shadow-essence of a Dark Templar warrior fused into a metal body.
Stalkers have been credited with many fantastic powers since their battlefield début. (as early as 2502). However, only one is witnessed consistently: an ability to instantly teleport, or "blink", from one spot to another. This ability requires augmentation to a Stalker, but the improved mobility allows the stalker to conduct ambushes, catch fleeing foes, and generally engage on their own terms.
Stalkers are often used to support Zealots on the battlefield.
In the game series, the Stalker is a Protoss ground support unit.
For more information see Stalker on the parent site.