Most hand guns have a trigger and a hammer, although I believe some rapid fire weapons can use the action from one discharge to fire another. The old pistols often had a "cylinder" into which the cartridges were inserted by hand. It would automatically index with each firing to bring the next cartridge into position. I believe most assault weapons have the cartridges in removable clip
The diagram is pretty helpful for listing the parts of the gun, like the barrel which is the long rod the bullet is fired from, the slide which runs along the top of the barrel housing, the grip and trigger set-up etc.