In StarCraft, Tarsonis is the capital world of the Terran Confederacy.
Tarsonis is one of the most important human planets in the original game's political history. As the Confederate capital, it represents the authority of the dominant Terran government that controls the Koprulu sector. The Zerg invasion of Tarsonis becomes a turning point in the first campaign and helps destroy the old Confederate order.
Mar Sara is a separate Terran colony where Jim Raynor begins the story. Korhal is the homeworld associated with Arcturus Mengsk and the Sons of Korhal, while Antiga Prime is another colony involved in the rebellion. These locations are closely connected to the campaign, which makes them common sources of confusion.
The fall of Tarsonis allows Mengsk to establish the Terran Dominion. The event also has lasting consequences for Sarah Kerrigan, whom Mengsk abandons during the evacuation. That betrayal becomes a major part of Kerrigan's later transformation and motivations.