A semi automatic pistol, semi auto for short, is a magazine fed handgun that uses some of the energy from the fired round to extract the empty case, chamber a new round, and often cock the pistol.
Most often the magazine is in the grip, but other locations have been used.
The less powerful pistols (mostly .22 rimfire) operate on the blowback principle. In this case the breech does not lock and is held closed only by a spring. When the gun is fired the rearward movement of the empty casing forces the breech block back against its spring, starting the cycle. The momentum of the breech block fully compresses the spring. Then the spring drives the breech block forward again completing the cycle.
More powerful pistols have a locking breech. In this case the breech remains closed long enough for the high chamber pressure to subside in order to prevent damage to the gun and injury to the shooter. There are several mechanisms to accomplish this but the most famous, and most copied, is the Colt model 1911 designed by John Moses Browning.
In this design the barrel is locked to the slide, which is integral with the breech block. When the pistol is fired the barrel and slide/breech block move rearward together, still locked, This allows time for the chamber pressure to drop. Then a toggle link (or a cam or a ramp in similar designs) unlocks the barrel from the slide. At this point the slide continues to move to the rear while the barrel stops. The empty case is extracted and the spring closes and locks the slide again.
A semi auto without a round in the chamber cannot be fired unless the action is worked to strip a round from the magazine and chamber it. Never the less, I would consider a semi auto to be loaded if the magazine contained any live rounds. A gun with a live round in the chamber is also a loaded gun even if the magazine is empty or absent.
The magazine release is often a button on the left side of the frame forward of the grip, near the trigger guard. On some pistols the button can be swapped to the right side. Others are ambidextrous. The release might be at the bottom of the magazine, or part of the trigger guard. Read the manual.
Note: You must do this in the correct order. Magazine out, then slide back. Otherwise, you risk ejecting one round only to chamber another if the slide goes forward again.
See states of readiness for a discussion of various conditions in which firearms can be placed.
In double action mode:
In single action mode:
Not all double action semi autos can be fired in single action mode. Those that can't are called double action only or DAO.
Some semi autos have a de-cocker. A de-cocker is a device that will lower the hammer without firing the gun. Other pistols require releasing the hammer with the trigger. Read the manual.
if the hammer drops on a live round the gun will fire. The hammer must
be lowered in a controlled manner while pulling the trigger, or with the de-cocker. Mechanisms can fail. Keep your gun pointed in a safe direction at all times!
If your gun does not have a de-cocker, put a thumb or finger of one hand under the hammer, while pulling the trigger and controlling the hammer with the other hand. Ease your finger out and let the hammer down gently. Keep the gun pointed in a safe direction at all times!
If you don't need to lower the hammer on a live round, it is better to unload the pistol before lowering the hammer.