An Iconian gateway or Stargate was a nearly instantaneous transportation device invented by the ancient Iconians. By stepping through the "door" or "doorway", the user could travel between two destinations thousands of light years away in a matter of seconds. Various types and designs of gateway were in existance.
One gateway had a range of over 70,000 light years, and theory postulated the Iconians used the gateway network as a preferred method of galactic travel. Federation scientists believed the gateways may acquire locations automatically, but remain unsure as the technology was significantly advanced. Energy weapons could not be used near some gateways due to interference.
The Iconians built their gateways by using an artifact they called the Dream Gem, and technology left behind by the Preservers. The USS Pendragon discovered several gateways during its exploration of the galactic rim in the 2383. Timothy Sinclair used one to transport himself to an alternate universe the following year, and still more were discovered in the Badlands in late 2385. (Star Trek: Pendragon: "Starkindler", "Of Shadows and Starlight", "Odyssey, Parts I & II", "Above the Wrecks of Time")
The first gateway encountered by Starfleet was found on the barren, ravaged surface of Iconia in 2366. USS Enterprise-D personnel Data, Worf and Jean-Luc Picard used the gateway to return to their ship in orbit. Before he left, Picard destroyed the gateway to prevent the Romulans gaining control of it. (TNG: "Contagion")
In 2372, the Dominion discovered another gateway on Gamma Quadrant planet Vandros IV. Deep Space 9 commander Benjamin Sisko co-led a joint Starfleet-Jem'Hadar team to defeat a rogue Jem'Hadar faction in control of the gateway. The team successfully destroyed the gateway. (DS9: "To the Death")
In one mirror universe variation the Terran Empire sent a fleet of starships to secure a gateway during a war with the Klingon-Cardassian Alliance. This backfired, as the planet launched probes that attacked the ships' computers and disabled the invasion force. What the Empire had left wasn't enough to defend itself, leading to its defeat (The Gift).