For this thread, I will define the basic nomenclature and terms you will run across while dealing with semi-automatic handguns.
SA (Single Action): This is a semi-auto that can only fire when the hammer or striker is in the cocked position.
DA (Double Action): This is a semi-auto that can only fire when the hammer is in an un-cocked position. In other words, the trigger pull cocks and releases the hammer in one pull.
DA/SA (Double Action/Single Action): This is a semi-auto that can fire in either firing mode. Most automatics are in this configuration.
Manual Safety: This means that the mechanical safety of the handgun must be manually manipulated by the handler of the weapon.
Decocker Safety: This is a specific type of manual safety that will drop the hammer safely when pressed.
Extractor: Commonly confused with the ejector, this is the part of the semi-auto that actually extracts the fired cartridge from the chamber during the firing cycle.
Ejector: This is the part of the weapon that ejects the spent cartridge from the weapon itself (the extractor takes the cartridge from the chamber, and the ejector ejects the cartridge from the weapon).
Safe-Action Trigger: A safety device on the trigger that will not allow the weapon to fire unless the trigger is correctly pulled.
Slide-Stop/Slide-Release Lever: A small lever in the side of the weapon that allows the slide to be locked in the open position and returned to the closed position. This lever is not absolutely needed for proper operation of the weapon, but is essential for fast reloads and certain clearing drills.
Magazine Safety: This is an internal safety that will not allow the handgun to fire when the magazine is removed.
Magazine Release: The button located on the side of the grip (behind the trigger) or on the bottom of the grip (less common) that releases the magazine from the weapon
Trigger Guard: The portion of the frame that extends over the trigger from the grip to the dust cover.
Dust Cover: The portion of the frame in front of the trigger guard.
Accessory Rail: Located on the Dust Cover, this rail provide a mounting spot for aftermarket tactical accessories such as flashlights, laser aiming devices, etc...
Loaded Chamber Indicator: A device that will show by either visual sight, or tactile touch whether or not the chamber has a live round loaded or not.
This is a basic list for now. This does not cover the internals of the semi-automatic weapon. This list is intended for the novice to be able to understand the "lingo" that is normally used around the gun counter at a retail store or at a shooting range.
Please feel free to add to this list.
God bless, stay trained, and stay safe.