In common speech, people often refer to "bullets" when what they really mean is "cartridge." You see, the bullet--the actual projectile that you're sending downrange--is only a part of a larger whole that's called a cartridge, and most of the cartridge actually stays behind when you squeeze the trigger. Some of it is transformed into energy and smoke, while some of it is left behind in the form of a brass or steel shell that is either ejected by the gun automatically, or by the shooter manually.
In great discussion from our friends at NRA Women, we're talking about rifle and pistol ammunition, which is a bit different from the ammunition we use in shotguns. In it, you'll learn about how the components of a cartridge function to cause your bullet to make impact at the target. Ammunition awareness makes us all better, safer shooters. Enjoy!