When you pull the trigger on a gun, the trigger releases what's called a hammer, which hits the back of a cartridge, which catapults a bullet out into the air.
In everyday language, people often call the whole package a "bullet," but what is pictured above is technically called ammunition or a cartridge. In technical jargon, the bullet refers to just the projectile part that flies out of the front of the gun. Normally that part does not contain any explosives.
The cartridges Pearlman photographed drastically change, depending on the bullets they're packed with: tiny ball-bearings, miniature arrows (which can penetrate body armour), or buckshot, to name a few. Below the projectile bullet is the propellant, commonly called powder because it is often gunpowder. Surrounding the ammo is the case.
At the bottom of each cartridge pictured is what's called a primer. When the hammer hits the cartridge the impact ignites the primer, when then lights the propellant. That energy forces out the bullet (projectile).