Types of fuel: Glow and gas(oline).

Glow is made up of methanol, nitromethane and oil whether the engine type of 2-stroke or 4-stroke.
Gas is made up of pump gasoline and oil for 2-strokes.

Ignition for a glow engine is a "glow plug". Looks like a tiny spark plug however it has a platinum wire coil that constantly glows red hot and provides for ignition on the compression cycle.
Ignition for a gas engine is a spark plug and uses an ignition system to provide spark.
Yea, there's a diesel but I'm clueless about 'em.

Sizes of glow is typically cubic inches from .02 to 1.60 and above. When you see someone refer to a 60-sized plane they are referencing the size of a glow 2-stroke that will adequately power the plane (OS 61 FX, OS 65 AX, etc.). I'm gonna guess on this one - an equivalent powered 4-stroke is about 125% the size of a 2-stroke (maybe higher percentage).
Sizes of gas is typically cubic centimeters are typically big (until recently) 50 cc, 100 cc,

Mess factor of glow is significant as there is a ton of post ignition residue that exits the exhaust and ends up as a slimy mess on the plane's covering.
Mess factor of gas is non-existent.

HTH

And now for the expert input ...