I was confused by the title so I had to look it up.
Generally speaking, a pistol is a handgun and a handgun is a pistol.
http://www.differencebetween.net/object/difference-between-pistol-and-handgun/
You have to get into the weeds to really come up with a difference. i.e. this is not the “clip vs magazine” sort of thing.
All pistols are handguns, but not all handguns are pistols. Technically, a revolver is not a pistol.
Used to be in TX when a person got their CHL that if they qualified with a wheelgun they were only authorized to carry a wheelgun but if they qualified with a semi they were authorized to carry either a semi or a wheelgun.
I say wheel gun instead of revolver just because it’s a cool word.