there are infinite types of relays..but all are basically switches.
for example: imagine yourself flicking on/off a housewall switch. the household switch by itself won't turn on or off by itself-it requires an external source (in this case a finger) mechanical, electrical or both, to actuate the switch, so that it can complete the circuit inside the wall.
almost the same concept applies to common automotive relays- it requires [not a finger 
] electricity to move its internal switches.
when electricity is applied to the internal coil of wire, a magnetic field is created, thereby mechanically pulling or pushing its magnetic arm that acts as a internal switch.
so.. if you see a four (or more) pronged relay, two of its prongs are almost always dedicated to +12vdc and ground(chassis). the other two (or more) are for whatever circuit completion(passive or active applications).
