Warning: Wikipedia contains spoilers.
Originally a Cardassian mining station named Terok Nor in orbit around Bajor, it was abandoned and wrecked at the end of the Cardassian Occupation[?] of Bajor. It was placed under joint Starfleet and Bajoran control and renamed Deep Space Nine (DS9). Subsequent to the discovery of the Bajoran wormhole, it is moved to the mouth of the wormhole to consolidate Bajor's claim to it.
Physically, the station is composed of a broad outer Docking Ring, an inner Habitat Ring, and a central core containing the fusion reactors, the Promenade public area, and Ops, the command centre. Sweeping up and down from the Docking Ring and defining a spherical shape are three sets of docking pylons. The station is over a kilometre in diameter.
The station serves as a base for the exploration of the Gamma quadrant[?] via the wormhole, and is a centre of trade and travel for the denizens of the sector. It is the home port of a number of Starfleet runabouts; the U.S.S. Defiant, an experimental warship, is added to its complement later in the series.
The station and its crew play a central role to the intense politics surrounding the Bajoran sector, the wormhole, and their environs. After the encounter with the Dominion and owing to the volatile conditions, the station is retrofitted and heavily armed.
According to The Star Trek Encyclopedia, 2nd ed.: