They are ruled by Chella, daughter of Cheyk. Together with other hill tribes, the Black Ears joined Tyrion Lannister and helped him get out of the Vale in return for money and weapons to take over the Vale for themselves.
In the books
The Moon Brothers and the Black Ears have strong bonds together.
While the Black Ears do cut the ears off of their fallen enemies, they don't necessarily kill them. As Chella explained: "We don't kill our victims. Only cowards do so, because they fear they can't conquer them twice."