The United States legal code USC 18 defines the shotgun as "a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger."
This definition, however, does not exactly match the colloquial use of the term, which would include the growing number of shotguns specifically designed to fire single projectiles ("shotgun slugs"), which have a rifled barrel rather than a smooth bore.
Also, colloquially, many people would likely call a fully automatic shotgun a shotgun, even though legally it would fall under a different category.
There are many types of shotguns, and they are a perennial favorite with collectors. There is the over and under shotgun, the side-by-side shotgun, both of which are types of double barreled shotguns. There are [pump action shotgun]?s and [semi-automatic shotgun]?s.
In hunting circles, the shotgun is used primarily for bird hunting, although it is also increasingly used in deer hunting in semi-populated areas where the long-distance effectiveness of the rifle may pose too great a hazard.
The shotgun is also known as the premier weapon for home defense. It is particularly suitable for this purpose because it is very intimidating (making it less likely that actual gunfire will be needed to end a criminal attack) and will often end the assault with a single shot, if it comes to that.