Ignition coils are really one of the best examples of just how much scientific ingenuity has gone into the design of the modern automobile. As an integral part of the way a car turns low voltage battery power into more useful high-voltage current, it is an integral part of how any car functions.
The primary operational pieces of ignition coils are the breaker, a capacitor, a primary winding, and a secondary winding. When the breaker is closed, energy from the battery builds up in the winding. This is only possible because of the winding being wrapped around the capacitor, which creates an inductor-- or something known as a "tuned circuit". As the energy builds, the breaker releases and the energy flows more freely, however that's not where it all ends. The electrical flow between the windings and the capacitor create a variable electric field that greatly enhances the voltage of the initial energy input from the battery. This is used to enhance the life of the battery, to enhance the output of the engine, and to provide enough power to operate the many "non-essential" components in modern cars.
Ignition coils are found near the spark plugs in most modern cars, and generally "between" two or more. That is, in a four-cylinder engine the ignition coils will be between two spark plugs, generating the electrical inputs for both spark plugs. They are small, cylindrical components that are generally very easy to distinguish, and they are connected directly to the car battery.