Types of Ammo: Shot
Cartridges filled with shot are the most common type of shotgun ammo. Shot are little balls made of any number of metals, including lead, steel, bismuth, tin and zinc.
Each metal behaves a little differently. Lead has some properties that make it one of the most effective materials for shooting game and targets. It is relatively heavy and therefore maintains its explosive force well. It is also somewhat soft, so it changes its shape as it leaves the barrel. This gives it a more spread-out shot pattern than other materials but still delivers a great deal of energy. There is some evidence that because steel pellets do not deform -- they maintain their round shape throughout their flight -- they wound animals without killing them more often than lead.
Until the early 1990s, most shot was made of lead. As environmentalists studied its effect on the ecosphere, they found that the spent lead shot hunters left in waterways and forests had harmful effects on wildlife and risked contamination of drinking water. Lead shot has been banned from waterfowl hunting in the United States since 1992, and various types of steel and alloy shot have taken its place.
The rule of thumb for shot size is the higher the number, the smaller the diameter of the shot. There is a consistent standard in the United States, but worldwide the numbers don't correspond to any specific measurement across the board. At Chuck Hawks' Shot Pellet Information and Recommendations, you'll find a guide to the various sizes in the United States and what they are used for. In hunting, smaller ammo is used for smaller game, and larger ammo is used for larger game. Buckshot is large-sized shot that got its name because it is used to hunt deer. Because different materials have different weights and characteristics, shot size alone does not tell the whole story. For example, if you are shooting with steel, you'd have to use larger shot than you would if you were doing the same type of hunting with lead.
Types of Ammo: Slugs
Slugs are molded chunks of metal, nylon or plastic. In effect, they turn a shotgun into a crude rifle. Slugs are fired individually, like bullets, instead of in bunches like buckshot and birdshot. They can come in a variety of shapes, but they are often tapered into a bullet shape. They can be solid or filled with substances like explosives or incendiary powder.
Shotgun slugs can be rifled -- this is supposed to make them spin in the air and thus improve their flight length and accuracy.
One reason hunters use slugs is to hunt deer in states that ban the use of rifles and/or buckshot ammo. The shotgun/slug combination provides a legal, if shorter range alternative. There are at least 20 states that have restrictions of this kind.
Types of Ammo: Sabots
A sabot is a specially shaped, two-stage cartridge. It has an outer jacket that helps it travel longer distances, and it has an inner slug or payload. The jacket is designed to fall away in flight after it reaches a certain distance. Several hunting sources suggest that sabot ammunition is only effective at longer distances when shot through a rifled barrel. For a shotgun hunter, this usually means adding on a rifled choke tube.
Sabot can also describe an arrow-like shape of material that fits in a standard shell. One particularly frightening sabot-style payload is the flechette. A flechette round contains hundreds of small, needle- or razor-like projectiles designed to penetrate armor and inflict painful wounds. They are banned by the Geneva Convention but do still see use in combat and counter-terrorism from time to time.
Types of ammo: Breaching rounds
Shotguns are commonly used in the military to "unlock" doors when troops don't know what lies on the other side. Because traditional ammo tends to ricochet and may end up hitting the shooter or someone inside the room, breakable "breaching rounds" are often used. These shells contain a metallic powder that disperses on contact.
This is all from www.howstuffworks.com