Here's a simple, easy to digest relay definition. For the most part, relays use lower voltages and currents to control larger voltages and currents, otherwise all the switches and buttons in our cars would have to be industrial grade and HUGE!! to handle current loads. Transistors are in a way like relays. They use a small voltage to control or valve larger currents and voltages. Relays can be simple or very complex, such as the OVP item in the MBZ's. As Steve stated this device "clamps" voltage to sensitive devices with a zener diode, which is a semiconductor that only passes it's rated voltage. Over that number, the fuse blows and the relay opens and protects the sensitive electronics from being baked.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.