High templar are support units with powerful abilities. They are not very durable and do not have an attack, not to mention their slow movement. Although high templar hover, they still activate spider mines.
The high templar selects a unit, creating two duplicates of it. Hallucinations appear with a blue tint to the creating and controlling player, while all other players will see them as duplicates of the original. Hallucinations do not inflict damage when they "attack", may not cast spells, or produce units (important for hallucinated carriers and reavers), and take double damage from all attacks. Hallucinations last for a limited time and automatically disappear when hit by spells (eg. EMP shockwave, Mind Control).
High templar had a direct attack called psi assault during development. Some high-templar-based heroic units (Tassadar) retain it. During the StarCraft beta, three high templar were required to make an archon.
For StarCraft: Remastered, the high templar's unit profile was changed to have blue eyes. Fan backlash was so extreme that the change was reverted.