The semi-automatic handgun is a magazine fed, recoil operated gun held in your hand.
Semi-automatic is just what the name implies – With each press of the trigger, the handgun fires a single round, ejects the spent case, grabs a new round from the magazine, then inserts it into the breech of the barrel ready for the next trigger press. The handgun requires the trigger be pressed each time to fire a round.
Semi-automatic handguns and revolvers are very different with each one have advantages and disadvantages over the other. Check out my articles on choosing a handgun and revolvers for more information.