What are the Cities of Refuge?

In an upcoming section of the Torah, we read about a unique biblical establishment: the cities of refuge.

These were places where those convicted of an accidental killing were sent. A city of refuge provided protection from retribution by the victim’s family. 

Our ancestors needed cities of refuge because of the cycle of violence. If a man killed someone, even in an accident, the victim’s family would seek revenge. This was common practice, as it still is in some parts of the world today.  

Our legal system has advanced since that time. We have the concept of manslaughter, an accidental killing, where the punishment is less. But we also still have cities of refuge. What are they?