The ignition coil generates the "spark" required to ignite gasoline. The ensuing explosion is part of the process the engine uses to create power. Older vehicles use only 1 ignition coil and a distributor to distribute the spark to each cylinder in the engine. Most modern vehicles use one ignition coil per engine cylinder. The ignition coils on these vehicles are located directly on top of the spark plug.
To replace the ignition coil, the electrical connection to the coil is disconnected and the hardware securing the ignition coil to the valve cover or manifold is removed. On some vehicles, it may be necessary to remove the intake manifold to gain access to the ignition coils.