distributor cap and wires
If you have an older vehicle with a distributor you should probably change the cap and rotor whenever you change the spark plugs. These parts are an integral part of the ignition system of your car, and could leave you stranded if they fail.
As the engine runs, the rotor is turned by the distributor. As the rotor turns, it touches leads on the distributor cap, transferring the high energy from the ignition coil to the spark plugs as it touches each lead. Because the rotor turns with the engine and rubs against the cap, both parts will eventually wear out. These days fewer people are learning how to change a distributor cap and rotor; however, it used to be a very common task and is quite simple.
The distributor is normally located directly at the front or back of the engine (in a v-type engine it will usually be located between the two banks of cylinders). With the engine off, leave the spark plug wires attached and remove the distributor cap by undoing the clips or removing the bolts that hold it in place. Carefully move the wires one at a time from the old distributor cap to the new one, making sure that the are going on in the same locations as they were on the old cap. The wires should come off fairly easily by grabbing the boot and pulling while giving a slight twist. If the wires get switched around the engine will run roughly and damage may occur. You may wish to apply some dielectric grease sparingly to the boots of the spark plug wires as you reconnect them to keep moisture out. Once all the wires have been swapped over, the rotor can be removed by pulling it straight off. Try to remember its orientation; the rotor should only go on one way but this will make installation a bit easier. Replace the distributor cap in the same orientation it was originally, and re-secure it with the clamps or bolts.
Once everything has been replaced and reassembled, start the car. If it isn't running smoothly shut off the engine and verify the wire routing and make sure that the cap is secure. If the engine seems to be running properly take the vehicle for a test drive to ensure that it runs well throughout the rev range.