Tribe is OK, though "people" is preferred now. Within many peoples there is also a clan system. The one I am most familiar with is that of the Baganda people of Uganda. My wife is a Muganda (the word for one member of the Baganda people) and is a member of one clan, I am white but have been invested into a different clan. Because of this, I can also call myself a Muganda. Intra-clan marriage is frowned upon. Uganda has around twenty or thirty tribes, some of whom also use a clan system.