Fiocchi Large Pistol Primers FAQ:
A primer is part of the ammunition cartridge. There are four parts to your round: the primer, powder, bullet, and casing.
Primers typically contain lead azide. It reacts to other chemicals in the powder to create an explosion when it is struck with heat and force (the firing pin).
Every company uses different chemical compounds, which is what differentiates primers from each other. They can also be shaped differently inside, making each explosion occur a bit differently.