Can someone explain, in detail, how this circuit function?
This is supposed to provide over voltage protection for a system
  placed in a car.


It does and the level at which that over-voltage protection begins is about 23 to 30 volts. The resistor bias chain that feeds the BZT585B33T is quite high in value and this means the zener will start conducting significant current possibly as low as 23 volts. At some point between 23 volts and 30 volts, the BC857 will turn on and this will short the MOSFETs gate and source. This turns off that MOSFET and therefore disconnects the load thus protecting it.

Will this protect an average car radio should 20 volts be applied? No it won't but it will protect a lot of equipment against the hundred volt plus voltages seen during a load dump in an automobile: -