What is an aircraft magneto?
A magneto is a gear driven electric generation device connected to the crankshaft of the engine. It supplies the ignition system (spark plugs) with power. 

Each engine has two magnetos. Each cylinder has two plugs. Each magneto supplies one plug in each of the cylinders. This makes the ignition system redundant. If a magneto fails, the other magneto can sustain the engine. 

Since the rest of the aircraft's electrical system is supplied by a belt driven alternator/generator, the engine is electrically isolated from any other electrical issues. If you lose electric power in the cockpit, your engine will still run. If you have to shut off electricity because of a cockpit fire, your engine will still run. 

The scale of the image below is a little distorted. I had to get my phone pretty close to the oil filter for sufficient lighting.  

MAGNETO #1 mounted below and to the side of the oil filter on the back of the engine


STARTER mounted under the front of the engine, connected to the flywheel only when in use


ALTERNATOR mounted under the front of the engine, connected via an accessory belt to the crankshaft