From the introduction:
Welcome to the Koprulu sector of space. Where the Terrans, (what humanity is known as here) is almost entirely descended from a group of prison ships that went off course. The sector itself was originally terraformed long ago by powerful aliens older than the galaxy itself, and in seeding life they gave rise to two powerful alien races; The Zerg, a race that utilize biology as powerful as any technology and The Protoss, with technology second only to the ones that seeded life here long ago.
You have come here October 2500, just as the brood war ends. It was a bloody conflict involving the Zerg, the Protoss, and a Terran force from earth seeking to take the power of the alien races for themselves. The Zerg leader, the Queen of blades won the war and then left back to Zerg controlled space.