The Black Ears are one of the hill tribes that live in the Mountains of the Moon. They derive their name from their habit of chopping off the ears of their victims and wearing those as a necklace. As Chella, their leader, put it: 'We don't kill our victims. Only cowards do so, because they fear they can't conquer them twice.'
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.