In the original BSG, the 13 tribes of man originated on Kobol and as they advanced, they went on their various spacefaring ways.
For some reason that I can't remember, the thirteenth tribe went off in a completely different direction, and all contact was lost, other than in the ancient stories which Adama used to inspire the fleet to continue on in the search for Earth.
It's a bit like the Romulan exodus from Vulcan. It's shrouded in mystery, and much has been forgotten.
Anyways, something went badly wrong during the journey, and the thirteenth tribe ended up loosing all the knowledge and technology that allowed them (us) to travel the stars - which is why they are so much more advanced than we are, even though we had the same origins and capability. I think it had something to do with the colony making a crash landing onto Earth, although what the cause was...? I have no idea.
There is an obvious flaw in this, because if it went so wrong, how did any knowledge of Earth get back to the twelve tribes? Like i've said before though, I haven't seen old BSG in a long time, so i've forgotten a lot of stuff about it.
I assume the storyline for the new BSG is roughly the same though? It is based on the old one afterall...