Tribals are Surviving Humans who have built a home for themselves in this horrid wasteland. These people probably worship random objects to great gods... like a building.... There are some Tribes that are friendly, others that are hostile, and the rest is free game.
Notable Friendly tribes include the Ash Bear tribe... that's pretty much it.
Notable Hostile tribes include the Stone tribe, and the CotF tribe, and pretty much any other tribe you attack