A internal combustion engine is a type of thermal motor in which the a fuel is combusted in pistons, turbine blade or trough a propulsive nozzle.
Types
Most popularly used internal combustion engines are "gasoline" or "Otto-type" engines (engines using the Otto-cycle. Otto-type or spark-ignition engines use a spark (provided trough a spark plug, powered by a battery or magneto) to deflagrate the fuel. Batteries require electronics (ie printed circuit board), whereas magneto's use a magnet to immediatelly convert rotational energy to electricity. The latter are thus sturdier and preferred in most cases. Advanced systems using batteries can be more fuel-efficient though.
"Diesel engines" are a entirely different type of engine.[1][2][3][4] Diesel engines use the Diesel-cycle[5]. As clear in the cycle, the inlet only introduces pure air, and not a air/fuel mix. A additional, seperate, fuel inlet is also present, the outlet is the same as with Otto-engines. For their ignition, they rely on compression-ignition.
Note that contrary to popular belief, the 2 types are not restricted to run on either petrol (gasoline) or diesel. The diesel engine for example was initially designed to run on peanut oil (a type of biofuel. It is still capable of running on this biofuel (as well as many other biofuels) although some tweaking is generally necessary to circumvent some current limitations (since the initial Diesel-engine, Diesel engines produced today typically use direct or unit-direct injection rather than indirect injection). Gasoline engines are typically a lot more restrictive in regards to the fuels they can run on (principally only ethanol, some gaseous biofuels (wood gas, oxyhydrogen, hydrogen, nitrous oxide, ...).
Besides Otto-engines (often of the line-engine type), several other Otto-types too exist. An overview is given at Comparison of IC motors