Rimfire ammo, like 22 LR ammo and 17 hmr ammo have a hollow rim on the base of the cartridge that has priming compound spun into it before the cartridge is loaded. Rimfire ammo is then fired by a firing pin crushing the rim of the cartridge to ignite the priming compound and then the powder. Centerfire ammo has a primer that is manufactured separately and then inserted into the base of the cartridge during the manufacturing process. As the primer contains an anvil to press against a firing pin or striker, it can simply hit the center of the primer to ignite the priming compound and then the powder. The greatest advantage of centerfire ammo is that the fired casings can be fairly easily reloaded at home. Each caliber will be either rimfire or centerfire and you will not have the option of which to purchase in a given caliber.