The document discusses the key systems of an internal combustion engine, dividing them into primary, operating, and accessory systems. It defines the primary system as the compression system, which includes components like the piston, cylinder head, valves, and camshaft that compress the air-fuel mixture and convert combustion energy into mechanical motion. The operating systems perform other functions necessary for engine operation and include the fuel, cooling, ignition, and lubrication systems. Accessory systems are optional for engine function and include components like the power steering.