Stargate

Overview
The stargate is an advanced protoss structure. It provides a warp link strong enough to call ships to a planet's surface. A large amount of electric (or similar) energy is generated just prior to the ship being warped in, showing itself visibly.

The warships of the protoss fleet require a much stronger warp link in order to materialize than a standard gateway can provide. The immense stargate is capable of calling ships from high orbit or across immense distances to appear on a planet surface. The construction of one or more stargates at a protoss base is a sure sign that it has become a major staging point for an offensive.

StarCraft
The stargate is the production building for protoss combat air units.

StarCraft II
Stargates will be returning in StarCraft II in a presumably very similar role.

It no longer warps in the mothership.