A space station is an artificial structure in space, usually built to sustain life-forms. Space stations serve many purposes, including administrative, defensive, intelligence gathering, mining operations, and starship maintenance. Space stations are usually constructed in orbit of planets, but can also be built to be self-sustaining in deep space.
All of the major powers of the Alpha and Beta Quadrants have developed and built space stations throughout their territories and/or along their various borders. Border outposts are usually strongly armed in case of any conflict. Deep Space 9, despite being built by the Cardassians as a mining station, was adapted by Starfleet into a "floating fortress" following the discovery of the Bajoran wormhole and first contact with the Dominion. (DS9 episodes: "Emissary", "The Jem'Hadar", "The Way of the Warrior")