Warp Blades are Protoss weapons created by the Dark Templar and based on the Psi Blade. They are exponetially stronger than their derivative and are favored by the Dark Templar warriors, yet they are slower.
Perhaps the most notable differences between the Warp Blade and Psi Blade, besides their combat performance, is their appearances and users. Psi Blades are the weapon of choice for the titular Khalai Zealots, where they find great use in cutting foes to ribbons with rapid slashes. Psi Blades are often dual-wielded and built into the wrists of a Zealot's power suit. The blade itself is made from whitish blue energy in an elegant blade shape.
Warp Blades take most of the aforementioned Psi Blade's properties and replace them with their direct opposite. Their users, and creators, are Dark Templar warriors, who use them for assassinating enemies from the safety of their cloaking field. Often, the Dark Templar do not use power suits, rather wearing simple cloth garments and billowing capes, so the Warp Blade projector is its own device. Dark Templar wield only one blade (as per the rule of fairness) as opposed to the dual-wielding Zealots. The Warp Blade's blade is more chaotic than the Psi Blade, manifested as green energy beam that appears to be burning.
Blizzard had intended to have the single-blade design for Zeratul only. Due to time constraints, this could not happen and all Dark Templar shared Zeratul's sprite. This has not been changed since then.