When the original Colonials left Kobol, there were Thirteen Tribes of Man. The Thirteenth tribe actually left Kobol long before the other twelve, and for reasons known only to the them and the original Kobolians, they went in a completely different direction to the others, almost 180o degrees in the other direction.
When they went, they still maintained some limited contact with the other Tribes, although after a while this ended. When the Colonials landed in the Cyranus System, they destroyed all their technology, so if the Thirteenth Tribe were still communicating, there would have been no one to have heard them.
Or it might have been that the Thirteenth Tribe destroyed their technology long before.
The Thirteenth Tribe has mainly fallen into Legends, and the majority of the Colonials these days either have never heard of the tribe, or just view it as a fairy tale. However they are written about in some of the old books - the Book of the Word, and the Lords of Kobol. It is known that the tribe went to a planet called "Earth", but aside from that, not a lot is known.
One story, which seems to be truth, is that Earth was known to the people of Kobol before the exodus. They had sent out explorers who had found the planet, who realised that it was another paradise planet like Kobol. Unfortunately, this happened during the Ninth - and finally - Lord of Kobols reign. This Lord was a sceptic, and despite the fact that he could see what was happening around him with his own eyes, he refused to believe that his planet was doomed, and thus did not understand the significance that Earth represented. He thus had the scout crew killed on their return, and kept the location secret.
When the Thirteenth Tribe left, they had either discovered the location of Earth, or else they set out to find it, following the path of the doomed scout ship.
After the war, the Galactica will arrive at Kobol and find proof that the tribe existed, and will almost uncover the exact location as well. However a Cylon raid would keep that information from them.
What happend to the tribe, what their level of technology is like, who they are, and in fact any other details of them, are not known by anyone.
[Obviously in the series Galactica 1980 the Galactica found Earth, which was set in the year 1980. However from the original musings from Glen Larson, that was never going to happen and when Earth was found, it was going to be a future setting. The current books (and indeed the comics) ignore Galactica 1980, so if you want to set a campaign in the future where the tribe is found, feel free to do what you like as they are no canon details to contradict.]