This is a modern double-action revolver:

 

Double Action - Single Action Revolver with the hammer cocked

Double Action – Single Action Revolver with the hammer cocked

This is a semi-automatic pistol.

 

Semi-automatic pistol, Sig Sauer Mosquito

Semi-automatic pistol, Sig Sauer Mosquito

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.

Double action only revolver with cylinder open. Ammunition goes in the holes (chambers) in the cylinder

Double action only revolver with cylinder open. Ammunition goes in the holes (chambers) in the cylinder

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.

 

Semi-automatic with magazine removed. Note ammunition in top of magazine

Semi-automatic with magazine removed. Note ammunition in top of magazine