According to Battlestar Galactica legend, the original thirteen colonies left Kobol when they could not peacefully coexist with the gods. Twelve colonies traveled into deep space to a solar system large enough to allow each tribe their own planet: each named for a sign of the Zodiac.
The thirteenth tribe struck out on their own and eventually settled on Earth. It is the search for this lost tribe, and the promise of escape from the relentless pursuit by their mortal enemies, the Cylons, that gives hope to the remaining survivors of humanity.
Here's a breakdown of each colony, the corresponding constellation on Earth and the most significant members of the tribe.