How does a Cessna's engine work?

Cessna currently makes both piston-engined and turbine-powered aircraft.

Assuming you mean a piston engined cessna, the engine works basically the same way as the engine in your car. Most small piston-engined aircraft use an air-cooled opposing 4 or 6 cylinder engine very similar to the engine in an old air-cooled Volkswagen.

Aside from that the other major differences are that aircraft engines usually use a solid-state magneto instead of an ignition coil, and that aircraft engines allow the operator to adjust fuel mixture levels en-route. The magneto is used so that the ignition system can be made independent of the aircraft's electrical system. Even if the electrical system fails entirely, the engine will keep running. The mixture control allows the pilot to adjust fuel mixture in-flight in order to compensate for the lower air pressures at high altitudes. If the aircraft were run at the same mixture all the time, as an auto generally does, the spark plugs would quickly become fouled by carbon and ash deposits.