The Tribe are a small group of outside Nomes in Blackbury including Masklin, Grimma, Granny Morkie and their 'leader' Torrit, having lost most of their number to natural causes, which to something Nome sized could be said to include anything with sharp teeth.
They probably would have died out if Masklin hadn't convinced them to leave their home by truck, which took them to the Store (the Nome paradise on earth with over two thousand inhabitants) where the Thing (an ancient computer the tribe had been keeping as a sort of good luck charm for thousands of years) finally came back online. It then instructed the Nomes in how to get out of the Store on board a truck after inspecting transmissions about its in impending destruction.
The members of the Tribe are probably the four most important Nomes that ever lived, as Masklin was the Nome that went to Florida with the Thing to bring back their ship from the moon, finally allowing Nomes to leave Earth.
The Tribe are also much tougher than store nomes, having adapted to the life Outside. Even Granny Morkie was able to drive off bandits, mainly by giving them the sharp side of her tongue.