Your standard DIN relay (the Bosch/Hella ones that are used all over the XJ - they are about a cubic inch in size) has five prongs. The are numbered 30, 85, 86, 87, and 87a.
Pin 30 is used for the power supply to the accessory. Typically, the fuse goes between the relay and the source (so the lead to pin 30 has a fuse inline.)
Pins 85 and 86 are for the coil (the part that pulls the contacts together) - and they are not poarised. One will go to a ground, the other to a switched power source (my convention is to ground pin 85, and put the switched power to pin 86.)
Pin 87 is a "Normally Open" contact that is closed when the relay is powered - that's the one you're going to use.
Pin 87a is a "Normally Closed" contact that is opened when the relay is powered - which allows you to manually turn something OFF when it's normally on. Pin 87a isn't used a lot.
You can also find DIN relays with two pin 87 and no pin 87a - these are "DPST" relays, and are usually used to power two circuits (like two lights, when you want to run individual leads to them.)
Relays (and switches) may be used for anything below the ratings given - you don't need to run a full 12VDC (nominal) through a relay. You can use them to switch microvolt signals, but it's kind of pointless (microvolt signals are typically also microampere signals.) However, if you've got something that draws three volts at a dozen amps, a DIN relay will work for you there as well. You can always work below a rating, never work above a rating!
If you get true DIN relays (like from Bosch - I use theirs exclusively,) you'll have a "reminder" of the internal arrangement and terminal functions mounted in one side of the relay cover.
Mainly, what I need to know is how many lights of what wattage you're running, and if you want to be able to turn some of them on independently, or all at once (like, say, turn on the end two or middle three, or are you powering all five at once? It makes a difference in wiring.
Again:
Pin 30 - Fused lead from source voltage (usually the battery.)
Pin 85 - Ground
Pin 86 - Switched Low-Current power (from your toggle switch)
Pin 87 - To Accessory
Pin 87a - No connection.