The main storyline of the StarCraft universe takes place in the Koprulu Sector.
The Koprulu Sector is a remote region of the Milky Way galaxy containing the worlds and colonies central to the conflict among the Terrans, Protoss, and Zerg. Major locations such as Aiur, Char, Korhal, and Tarsonis are located within or closely associated with this setting.
The sector’s Terran population descends from criminals and political dissidents transported from Earth in the 23rd century. Their colonies developed into competing powers, including the Terran Dominion, the Kel-Morian Combine, and the United Earth Directorate’s expeditionary forces.
The Koprulu Sector is not a fourth faction or a single planet. It is the broader geographic region in which the campaigns occur. Earth lies far away from it, which helps explain why the sector’s Terran societies developed independently before the UED intervened.