Shotgun ammunition is called a shotshell. Shotgun ammo is typically broken down into three categories: birdshot, buckshot, and slugs.
A shotshell consists of a cylindrical casing filled with metallic pellets. The casing is typically made of paper or plastic. This tube sits on top of a brass base which holds the gun powder. Inside of the casing, above the powder, there is usually a wad of paper or plastic material which holds the shot in the upper end of the case.
Rifles ammunition is called a cartridge (or round). A cartridge includes a metal casing, powder, primer and bullet.
The inside of a shotgun barrel (the shotgun's bore) is smooth.
The bore of a rifle has spiraled grooves (called rifling).
The smooth bore is the design element that allows a shotgun to shoot shells and have the shot disperse in a splatter pattern with the highest density of shot being in the middle of the pattern.
If a shotgun barrel had rifling, the rifling would cause centrifugal force on the shotshell. Once fired, the shot would disperse in a donut-shaped pattern with the highest density of shot around the edges. This would leave the lowest density of shot in the middle part of the donut-shape.
Shotguns are measured using gauge. The history of measuring by gauge dates back to muzzle-loaders. Gauge was determined by how many lead balls (that were the size of the gun's bore diameter) would make up a pound.
So, if 10 lead balls (whose diameter was "10") made up 1 pound, the shotgun was a 10-gauge shotgun. If 12 lead balls in a "12 diameter" were needed to add up to a pound, then the shotgun was a 12-gauge shotgun, of course now days we use a more precise way of measuring gauge.
This means that the smaller the shotgun gauge, the bigger the barrel and consequently, the bigger the ammunition.
The diameter of the barrels on handguns and rifles are measured using caliber.
When compared with a rifle, a shotgun is not as accurate.
However, someone new to shooting may find it easier to hit a moving target with a shotgun because of the way the ammunition (which is made up of multiple pellets sometimes 100's of round projectiles per shell) spreads out, this is why we see shotguns being used to hunt rabbits, ducks, and quail.