Blizzard have added a story page for all the lore back story of the StarCraft universe, up to StarCaft 2. It is the first part, in hopafully many more to come.
"Terrans first journeyed to the far edge of the galaxy when a navigational malfunction sent their automated colony vessels hurtling off course. Bypassing their intended target, the ships crash-landed on three worlds scattered across the Koprulu sector.
Over several decades the terrans established colonies, spread to other planets, and came into conflict with one another. In the decade after the end of the Guild Wars, the Terran Confederacy stood unchallenged in its supremacy over colonial space.
The terran colonies first became aware they were not alone in the universe when a gleaming fleet of protoss warships suddenly emerged over the Confederate world of Chau Sara. The advanced alien ships incinerated the planet's surface without warning."