It's a bit of a misnomer, as "autoloading" shotguns don't
actually load themselves. They reload themselves. Most autoloaders
have a tubular magazine
that will hold three or more shells. Combined with a shell in the chamber,
this means quite a lot of firepower.
Autoloading shotguns are either gas operated or recoil (inertia)
operated. In gas operation, a port in the barrel allows some of the rapidly
expanding gasses from the shell's firing to escape and operate a piston
that operates the automatic loading action. In an inertia/recoil system, the
force of shell being fired drives the action. Recoil operation, in a well-made
shotgun, is ultimately more reliable than gas operation, but can feel a little
more sharp to the shoulder. If a repeating gun is the right tool for the job, but
the shooter has any worries at all about the reliability of an autoloader (or the
price feels a little steep), a pump-action
gun is a solid choice, if not always as elegant.