Both are handguns. A revolver contains a revolving cylinder in which bullets are loaded. Revolvers usually hold six shots. The ATF defines a pistol as any handgun that does not contain its ammunition in a revolving cylinder.
The term "Pistol" refers to a handgun having one chamber integral with the barrel, making Pistolsdistinct from the other main type of handgun, the revolver, which has a revolving cylinder containing multiple chambers.