One point to add: if the test light shows pulsing power to the coil, and your ignition system uses points (e. g. non-electronic), before writing off the coil, test the condenser (by meter or by substitution). Without a good condenser, you cannot mechanically break the coil power fast enough to keep all of the coil energy from being dissipated in an arc at the points. [Obviously, electronic ignitions don't suffer from this problem, as transistors can switch off in nanoseconds.]
Note: DO NOT routinely replace condensers, as the new production units seem to be of rather poor quality.
Every car has an electrical system. Here's the place to learn all about it.