Every firearm can be described in terms of three main parts: the barrel, the frame, and the action. The barrel is a hollow, cylindrical piece of metal, much like a water pipe. The action refers to the moving parts of the firearm, and contains a mechinism to actually fire the gun by means of a trigger, and usually some means for loading and removing ammunition. The round of ammunition is placed by some means in the chamber, usually the portion of the action connected to the barrel. Some actions utilize a magazine, designed to feed multiple rounds of ammunition into the action. The frame holds the action and barrel, and also has any ergonomic features, such as a grip, stock, or forearm attached.
The ammunition fired by a shotgun is referred to, logically, as "shot." Shot are relatively small balls of metal, usually lead, that can vary in size from as small as .08" in diameter up to about .40". Shot greater than .20" in diameter is generally referred to as "buckshot" and generally only has hunting applications. Smaller shot is referred to as "birdshot," and the smaller sizes thereof are used in the shotgun sports that will be presented here. A shotgun cartridge, or shell, contains a primer, which ignites upon impact, the powder charge, which creates the gasses that propel the shot down the barrel as it burns, and the shot. The shot load is measured in ounces, and will contain hundreds of pellets per shell.
For simplicity, we shall define a shotgun as a long gun (a firearm designed to be fired from the shoulder) with a smooth (not rifled) barrel, capable of firing shot. The action may be semi-automatic, or self-loading, whereby the gas discharge or recoil of firing a shot automatically ejects any spent shell in the chamber and loads another round from the magazine and places it in the chamber. Slide or "pump" action shotguns require the action to be cycled, usually by pulling the foreend of the shotgun back and forward to eject any spent round and load an unfired round from the magazine. Break-barrel shotguns do not utilize a magazine, rather the chamber is integrated with the barrel, and the barrel is tilted down from the frame so the shooter can manually load ammunition into the chamber.
When the gun is fired, the hundreds of pellets are propelled out of the barrel. At the end of the barrel is an area of constriction called the "choke," which may be adjustable or fixed. The choke packs the pellets together as they leave the barrel. Once they leave the barrel, they begin to spread out from each other as they travel through the air, producing a "shot cloud." This cloud will expand as it travels (an inch of expansion per yard is a rule of thumb often used) so that a relatively large area will be effected by multiple projectiles at the same time. This is advantageous when you're trying to hit a relatively small or fast-moving target, which is the basis of the shotgun sports that follow.
All the shotgun sports are (or were originally) based in some way upon hunting situations or similar activities. Most of them were originally invented as ways for bird hunters to keep their skills up in the off season. These games developed into sports in their own right, and two of them are even Olympic events. All of the following sports have international organizations to promote the sport and to sanction matches.
This is perhaps the oldest of the shotguns sports, and the simplest for the beginner. It is based on live bird trapshooting, in which a pigeon or other bird is released from a trap and shot as it takes flight. In modern trapshooting, the pigeon is replaced by a fairly fragile disk of hardened clay-like material, launched from a spring-loaded arm. Although the disk is no longer made of clay, the term "clays" or "clay pigeons" is still used. They are launched from an in-ground bunker (the "trap") in front of the shooter. The shooter calls for the clay, usually by shouting "pull," and the "trapper" launches it. The shooter gets one shot at the clay. There are several variants of trapshooting, including double trap where two clays are launched simultaneously, and Olympic trap which has slightly different rules and layout to make the game more challenging.
This game is similar to trap, but rather than having one trap right in front of the shooter there are two houses, one low and one high, that launch clays across the line of site of the shooter rather than directly away from him. This game was developed by trapshooters who wanted a game that was more challenging and more closely emulated wingshooting situations. Targets may be launched from both houses in quick succession, requiring a shotgun capable of firing two shots quickly. Skeet is generally played with a double-barreled shotgun (usually an over and under, rather than side-by-side) or a semiautomatic.
This is sometimes referred to as "golf with a shotgun." This is a catch-all term for a shooting game with a variety of target sizes and presentations. Sporting clays courses are generally divided into stations with specific presentations, for example, a clay might be launched in your direction, followed by one several feet above you flying away from you. A variety of clays may be used, including "rabbits" which are designed to be shot as they roll along the ground rather than fly through the air. Sporting clays has become very popular in recent years, especially among shooters who have grown bored with the predictable target presentations and who like to get a little exercise walking from station to station. A common variant of sporting clays is called "5-stand," which involves sporting clays presentations of targets but all the walking that's required is between 5 shooting stations situated closely to each other. Often two targets will be presented in quick succession, so this is also a game well suited to a double-barreled or semiautomatic shotgun. Another evolution of sporting clays is sometimes called a "flurry," in which many targets (20, 30 or more!) will be presented non-stop, and the shooter has to break as many as possible while overcoming the additional challenge of reloading.
This game is the most challenging of the shotgun sports. Unlike other games where the object is to break a clay target, ZZ birds are like little Frisbees with propellers. They are launched from a small trap that keeps the target spinning, and when launched the targets fly in a random direction due to the action of the propeller. After the target is launched, the shooter tries to hit it in such a way that the propellers break off, causing the target to fall to the ground. If the "witness ring," a small loop of plastic on the propeller, lands within a certain area, the target is ruled dead. If the witness ring falls outside the fair zone it's ruled a lost bird. Because there are many random elements involved, and the margin for error is so slim, this game is extremely difficult. Competitors are allowed two shots at each target, so this is another sport that calls for a double-barreled or semiautomatic gun. This game is far more popular in Europe than in the States, but its popularity is slowly picking up as an alternative to sporting clays.
Nothing is more important when dealing with firearms than safety. I highly recommend taking a firearms safety course, especially one that will cover shotgun operation and safety. Make sure you're familiar with the firearms laws in your area and comply with them to the best of your ability. Find a local club that offers shooting sports you're interested in, the odds are good there will be people there who are willing to introduce you to the sports. As mentioned above, most of the shotgun sports require a double-barreled or semiautomatic gun. The exception is of course trap.
A good option to try your hand at trapshooting for minimal cost (assuming you can't use someone else's gun) is to purchase an inexpensive or used slide-action or single-barreled shotgun. Mossberg pump shotguns are a good choice, and can be bought for under $200 with a 28" barrel that will serve adequately for trapshooting. A single-barrel New England Firearms shotgun with a 28" barrel can be bought for $100, or perhaps less if you find them on sale. The barrel should be at least 26" for trapshooting, any less will make breaking clays more difficult for you. Try to stick with a 12 gauge or 20 gauge gun, as shot is less expensive for those popular gauges.
Developing a proper gun mount on your shoulder and learning the sight picture of your gun varies from person to person and gun to gun. Hopefully you can find someone at a local club (or a friend or family member) to give you some pointers and tips. Once you start breaking clays, you'll find that you develop a point-and-shoot instinct. If trapshooting suits you, you'll get curious about shooting skeet and sporting clays, and perhaps decide to spend a few hundred dollars on a good sporting clays semiauto. You'll then be well on your way to developing a challenging and interesting hobby that's very addictive and rewarding.