I agree with carrellh, "nitro" engine is completely wrong so it's best not to get into the habit of calling them that. Their main fuel is methanol and nitro is just an additive which you can use or not. The proper common term is glow engine. This has always been one of my pet peeves and it seems to have started with advertising by the RC car people to make them sound powerful .

Basically there are 4 types of engines used in planes.
Glow engines (using methanol)
Gas engines (spark ignition using gasoline/petrol)
Diesel engines (using ether/kerosene/oil)
Gas turbines (whatever they'll run on )