A semi automatic handgun, often refered to as merely a pistol (regardless of the fact that this term encompasses several other types of small firearm) is most commonly used as a sidearm by police and military all over the world. Most types rely on a magazine which feeds bullets through the hollow hand grip into the firing chamber. This allows for quick reload times and larger number of bullets per loading than a revolver.
A semi-automatic weapon ejects the cartridge automatically once the bullet is fired without additional action by the user. This is accomplished by springs and gas pressure from the fired cartridge. A semi-automatic will fire only one shot per pull of the trigger as opposed to an "fully automatic" which continues to fire until the trigger is released or all cartridges have been fired.
Popular examples: Glock 9mm, Beretti 9mm, etc. Colt .45 caliber