Dark Templars are invisible Protoss warriors that are rejected by the Protoss who have laid claim on Aiur. They were casted out and banished for their differences from the regular Protoss and it's considered blasphemy if a Protoss from Aiur ever aligns themselves with the Dark Templar.
Dark Templars can merge with others to create a Dark Archon, and are unable to ever change back. However, Dark Archons can use a power called Mind Control to take over enemy units.
 Game Usage
 Protoss vs Terran
Dark Templar are used to harass Terran bases, often dropped by Shuttles to delay production while Protoss can take more expansions. A straight rush to Dark Templar is considered all-in against a Terran, because a failed attack will result in a massive disadvantage for the Protoss.
 Protoss vs Zerg
Dark Templar are ideal for killing Drones. Since Drones die in a single hit, the "our forces are under attack" warning will not be given and players can lose an entire base worth of Drones if they are not careful.
 Protoss vs Protoss
Like Drones, Probes also die in a single hit causing no warning. Dark Templar builds also provide map control against other Protoss.
Related ThreadsOfficial Blizzard Poll: Dark Templar Design - last post by @ Apr 11, 2009
dark templars and lurkers underated??? - last post by @ Feb 17, 2003