The ignition coil is the heart of an automotive spark ignition system, but is essentially just an electrical transformer. It consists of a primary coil made up of 100 to 150 turns of heavy copper wire and a secondary coil made up of 15,000 to 30,000 turns of fine copper wire.
The primary coil creates, or induces, an extremely high voltage in the secondary coil. This causes a spark in the spark plug, which ignites the mixture of fuel and air in the engine.
Federal law in the United States dictates that the ignition coil and ignition module on new vehicles are warranted for at least two years or 24,000 miles, whichever occurs first. The insulation in the ignition coil will degrade over time, but many coils are designed to last 100,000 miles or more.
You can test the resistance of the primary and secondary coil windings with an ohmmeter, with the battery lead to the ignition coil disconnected. You need to refer to the manufacturer's specification for your specific ignition coil, but typically primary coils have a resistance between 8,000 ohms and 20,000 ohms and secondary coils between 0.5 ohms and 2 ohms.