The Stargate (or Astria Portal in Ancient or Chappa'ai in Goa'uld) is a piece of technology that forms a stable wormhole between the originating stargate and one on another planet. They were created by a race called the Ancients, but the Goa'uld widely used them to take slaves to other worlds. They can be powered by anything from electricity to zero point energy in the form of a (ZPM). They are also capable of going to Stargates in other galaxies if given sufficient power. There were two known types of gates built by the Ancients (one for the Pegasus galaxy and one for the Milky Way galaxy). They were also built by the Tollana, Orlin, and the Ori (which was a supergate powered by a blackhole).
The Earth based Stargates have been operated either by the government of the United States of America or the government of Russia. The gates on Earth were found in Giza (brought there by Ra) and in Antarctica (probably by the Ancients themselves). To enter coordinates you use a device called a Dial Home Device (DHD).
The Protoss also make use of Stargates. They construct massive stargates on the battlefield routinely; These are used for transporting spacecraft quickly from the Protoss homeworld of Auir to the battlefield quickly. These are unlikely to be the same stargates invented by the Ancients; their appearance and function is more consistent with protoss technology. The Protoss must have either invented them independently or seen and reverse-engineered the stargates. (Starcraft franchise: Starcraft, Starcraft 2)
In the 31st century, it appears that Stargates have become more common. Superman and the Legion of Super-Heroes utilized a public hub of numbered 'gates', each of which led to a different planet. They seemed in appearance and function to be near-identical to present day Stargates. These are more similar to, and may in fact be, Ancient stargates. (Superman franchise: Legion of Superheroes)