Avox are essentially enslaved citizens from the Districts who have been sentenced as criminals and condemned to becoming servants for the people of The Capitol as punishment for whatever their crime was, and were probably arrested by normal Peacekeepers.
Avoxes are distinguishable by their white uniform and their lack of a tongue, and look very similar to each other. Avox are less than second class citizens and are reminiscent of a time when it was appropriate to treat human beings worse than an animal. As such, they are often victims of abuse mentally and physically.
List of known Avoxes: