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