Original: While the term "pistol" can be properly used to describe any handgun, it is common to refer to a single-shot or auto-loading handgun as a "pistol" and a revolver as a "revolver".
Simplified: Guns are also called "firearms". In military terms, the word "firearm" is usually used for the type of weapons that soldiers carry (pistols and rifles), while the word "gun" is used for larger guns that fire at higher speed. The weapons on warships and tanks are called "guns" or "turrets" (if placed on a rotating base), not "firearms".