This is a modern double-action revolver:
This is a semi-automatic pistol.
The main difference between a revolver and a semi-automatic pistol is how ammunition gets into position to be fired. In a revolver, each time the trigger is pulled, the cylinder rotates moving a new round to the firing position. In this picture, the revolver is shown with the cylinder open.
On the other hand, a semi-automatic holds the ammunition in a magazine. A magazine is like a Pez dispenser. It holds a stack of ammunition on top of a spring. Each time the trigger is pulled, an expended ammunition casing is ejected and a new cartridge is pulled off the top of the magazine and placed into the gun’s chamber for firing.