What is a relay?, hopefully this page will help you understand what one is, what it does, and how it works.

The above drawings are my attempt to show you what a relay is, the top left hand picture is what a standard relay that you would use looks like, it is usually in a black plastic case, and is square in shape, the tag on the top is for fixing it to a wall/post etc, you can sometimes get metal cased relays, these usually have only 3 terminals on the bottom, because it uses the metal casing for the coils earth, these are only good if you can mount the relay on a metal surface that is earthed to the rest of the van's chassis,
Those pin like things sticking out of the bottom are the terminals you use to connect the relay in to your circuit, the next picture (top right) shows the underside of the relay, you can see the layout of the pins now, you will also notice numbers stamped next to the pins, these are to tell you what each pin does, there is usually a schematic diagram of the relay's internals molded on the top of the relay, but this is only good if you understand what goes on inside a relay, (which is hopefully the purpose of this page),

The numbers next to the pins signify what connection to make... and are as follows, pin 85 is for the coil, (or electromagnet), pin 86 is also for the coil, it doesn't really matter which way you connect these pins up, the coil only needs power flowing through it for it to work, it doesn't matter which way it flows, (except on a metal cased 3 pin relay, you will only have one pin for the coil, the earthling of the coil is through the case), although it is normal practice to use pin 86 for the positive connection and pin 85 for the earth.
The other 2 pins are for the switch part of the relay, pin 30 is for the moving contact of the relay, so connect the live part of the circuit to this pin (the wire with the power down it all the time), pin 87 is for the fixed contact, connect the switched part of the circuit here, (the wire to receive the power only when the relay is operated), on a 4 pin relay it doesn't matter which way you connect the switched contact wires up, as all the switch does it connect the 2 wires together to complete the circuit, but if you have a 5 pin relay, things are a little different, the last picture above shows the internals of a 4 pin relay.

A 5 pin relay is known as a change over relay, the 5th pin is usually numbered 87a, it is for an extra contact in the switching part of the relay, the moving contact rests against this contact when the relay is off, (you must connect the "live all the time" wire to pin 30 in this type of relay, so you can take a wire from pin 87a to feed a circuit with power when the relay is in the off position), when you activate the 5 pin relay, again, the electromagnet pulls the moving contact towards itself, this time it disconnects from the first contact, and then connects to the second contact, so it kills the first circuit, and puts the power down the second circuit,

This picture shows a relay in action, place the mouse cursor over the picture (anywhere will do), and left click the mouse (like selecting a hyperlink), the relay should now be energized, and the bulb illuminated, left click again to de-energize the relays coil, (First time you click it, you will have to wait a few seconds for the new image to load, but after that it changes instantaneously, do it as many times as you like to understand what happens), what's going on then.... well it's like this, one of the reasons relays were invented was to switch higher current loads than a small switch can handle, the electromagnet of a relay pulls very little power, so you can use a small power source, and a small switch, you can put up to the rated current through the relays contacts with no problems, (a 30 amp rated switch is quite large and bulky), you don't even need to use the same power source for the relay and it's switched load, in the picture above i've used a relay with a coil rated for 1.5 volts.. i.e. a torch battery, and i'm using a 12 volt car battery to power the 12 volt bulb.

The positive wire going from the car battery is live all the time, and is connected to pin 30 on the relay, where the power flows up the moving contact, and stops, it has nowhere to go...yet, pin 87 is connected to one side of the bulb's filament, and it's earth side is connected to the negative terminal on the car battery, that's the switch side of the relay sorted.

Now to the coil side, pin 85 is connected to the negative side of the torch battery, and pin 86 is not connected to anything yet, the coil is dead, and the bulb is dim, when you click on the picture, I have connected the wire from pin 86 on the relay to the positive part of the torch battery, you will see the wires change colour, (this doesn't happen in real life unless you have overloaded the wires, in which case a fire will start very soon and burn everything down, that wouldn't happen if we had fitted fuses in line though :o),
Any way, the power from the torch battery now flows down the wire to pin 86, which is connected to one side of the electromagnet, the power flows through the electromagnet's coils and back to the torch battery's negative terminal, the power flowing through the coil creates a magnetic force, which pulls the moving contact towards it,  this contact tries to get to the coil, but it can't because the fixed contact is in it's way, it is forced against the fixed contact... completing the high current circuit.

The positive electricity from the car's battery that was stopped at the end of the moving contact, is now able to flow through the fixed contact,  it flows through pin 87, and reaches the bulb, where it flows across the bulbs filament, making it glow, and back down to the car battery's negative terminal, and that's how a relay works.