There are 4 components that make up all rifle cartridges. The bullet (1) , the case (2) , gun powder (3) and the primer(5). (4) is the rim of the cartridge used by the ejector to move a spent case from the chamber.
The primer is the component that ignites the powder inside the bullet casing when struck by the firing pin of the firearm causing the bullet to fire. This cartridge is called a center fire.
There are minor variations to this in existence. The cartridge you see above has what is called a boxer primer. Some older cartridges are equipped with Berdan primers which is similar to the drawing above but has two openings between the primer pocket (5) and the powder (3). Cases with Berdan primers should not be reloaded as most reloading dies are designed for boxer primer cases.
The drawing below shows the difference between rim fire and center fire cartridges. In rim fire the location of the primer is inside the case and dispersed around the rim of the cartridge but both are made up of the same 4 basic components.