A relay is used for switching high amp loads with a low amp switch. For example if you put running lights on a car ... you don't want 30AMP of juice going through that little switch on your dash board to complete the circuit. So the little switch on your dash board just switches a small current to the relay ... which in turn is a electro magnetic switch to supply the high AMP circuit to your lights. Relays can be used from some cool things too. I just installed Power Door locks in my Jeep using factory power latches and door lock switches out of a 1995 Dodge Caravan. Here is the relay setup I used for each door for a total of 4 relays to control both doors independently. I needed 4 relays since the latches require reversing polarity to lock and unlock.
