In the Star Trek fictional universe, Deep Space Nine (or DS9) is a space station. It is the primary setting for the television series Star Trek: Deep Space Nine.
The station serves as a base for the exploration of the Gamma Quadrant via the Bajoran wormhole, and is a hub of trade and travel for the sector's denizens. It is run by a joint crew of Starfleet and Bajoran officers, and it is the home port of a number of Starfleet runabouts and the USS Defiant. The station and its crew play a vital role to the events surrounding the Bajoran sector, the wormhole, and the Dominion War.
Contents
[hide]
1 Description
2 History
3 Mirror Universe
4 External links
5 References
[edit] Description
Deep Space Nine, over a kilometer in diameter, is composed of a broad outer docking ring; an inner habitat ring containing residences; and a central core containing the promenade, the fusion reactors, and the operations center, known as "ops". Three sets of docking pylons sweep up and down equidistantly around the docking ring, defining a spherical shape. It is of identical design to Empok Nor. In its current position at the mouth of the wormhole, the station is three hours away from Bajor by shuttle.
The promenade is the main public thoroughfare in which visitors and residents congregate, on which can be found Quark's Bar (the crew's most frequently-seen off-hours location), the infirmary, the replimat, a Bajoran temple, Garak's clothing shop, chief of security Odo's office, a mineral assay office, a candy kiosk, and, for a brief time during the first season, Keiko O'Brien's classroom. A weapons storage area is located on Level 5, Section 3 of the habitat ring ("Captive Pursuit"). There are normally about 300 permanent residents of the station, though the station can accommodate up to 7,000 ("Sanctuary").
According to The Star Trek Encyclopedia:
   

The station model was designed at the Star Trek: Deep Space Nine art department by Herman Zimmerman and Rick Sternbach. Contributing artists included Ricardo Delgado, Joseph Hodges, Nathan Crowley, Jim Martin, Rob Legato, Gary Hutzel, Mike Okuda, and executive producer Rick Berman. The miniature was fabricated by Tony Meininger.
   

[edit] History
Spoiler warning: Plot and/or ending details follow.
Originally a Cardassian mining and refinery station named "Terok Nor" in orbit around Bajor, it was built by slave Bajoran labor under Cardassian rule in 2351. The station was commanded by Gul Dukat, the last Cardassian prefect of Bajor. It was abandoned toward the end of the Cardassian occupation of Bajor in 2369; the Cardassians removed anything of value on their way out.
In 2369, the Bajorans assumed control of the station. The Bajoran provisional government requested the assistance of the United Federation of Planets, who renamed it Deep Space Nine (DS9) and placed Commander Benjamin Sisko in command. Despite the Federation presence, the station remains subject to Bajoran law. Shortly after Sisko assumes command, his crew discovers the Bajoran wormhole. The station is moved to the mouth of the wormhole to secure Bajor's claim to it, after which the station assumes great commercial, scientific and strategic importance.
After the encounter with the Dominion in 2371 and owing to other volatile conditions, the station is retrofitted and heavily armed. The station's full complement of weaponry includes 5,000 photon torpedoes, several phaser banks, and shields that extend 300 meters around the station. These upgrades are first used against the Klingons in their offensive against Cardassia in early 2372. At the beginning of the Dominion War in late 2373, the station is temporarily lost to the Dominion. Federation and Klingon forces retake it in early 2374 after the Battle of Bajor.
Aside from a failed Bajoran military coup, the Dominion's occupation and a brief leave of absence from late 2374 to early 2375, Sisko, promoted to captain in 2371, remains in command of Deep Space Nine. In late 2375, at the end of the Dominion War, Colonel Kira Nerys assumes command.
[edit] Mirror Universe
"Terok Nor" is also a space station in the Mirror Universe. Built under the authority of the Klingon-Cardassian Alliance in orbit of Bajor (because the wormhole is unknown there). In 2370, the station is the command post for the Alliance authority throughout the Bajoran sector, under the command of Intendant Kira Nerys. The station also serves as a processing center for uridium ore mined from Bajor; the processing facilities are manned by Terran slaves.
In 2372, the Terran Rebellion wrests control of the station away from the Alliance. The Alliance, determined to recapture the station, sends a fleet under Regent Worf to fight the rebels. The rebels are able to build their own Defiant based on schematics for the Defiant in the primary universe, and they use it to defeat the Alliance fleet. Terok Nor remains a base of operations for the Terran Rebellion for at least the next three years.
Spoilers end here.