There are only three valid reasons that horses are shod: protection, traction or to effect a therapeutic change in  the way a horse moves. All else is vanity.

Protection is the most obvious reason: Simply put, if a horse's rate of foot wear exceeds his rate of foot growth, his foot must be protected in some way. If not protected, continuing the same routine, in the same environment, will cause soreness. Once sore, the owner can either lay him up while he grows out enough to protect himself with hoof wall and exfoliating sole, or shoe him and accomplish almost the same thing artificially.

The most obvious form of protection is a horseshoe. But, what kind of horseshoe? Horseshoes are made from rubber, plastic, steel, aluminum, titanium and occasionally from other materials ranging from brass to rawhide.

The most common type is steel. Steel horseshoes are readily available and easiest to use. They can be easily modified or forged from barstock with only minimal forging skills. Steel is easily welded or brazed for specialty applications. Steel comes in a variety of widths, thickness and configurations. Many farriers die of old age without ever nailing on anything but steel shoes. Occasionally though, horses need other kinds of shoes.

Aluminum has found widespread acceptance in flat and harness racing, and on the fronts of speed event (e.g., barrel horses) where weight of the shoe is an important factor. Aluminum is also very popular on the fronts of hunters as many trainers feel it enhances a hunter's way of going. It is also the first choice of many veterinary farriers as its greater width and thickness (relative to steel) can be used for protection of sensitive structures without adding weight. Aluminum is not quite as easy to forge or weld as steel.

Solid rubber and plastic shoes have very limited specialty applications (e.g., horses used for drayage on hard surfaces, pathological conditions). Full (covering the sole) synthetic shoes trap moisture and create ideal conditions for Thrush; rim-type synthetic shoes simply don't stay where they are put: they have an embarrassing tendency to shift and extrude. Open, (sole not covered) rubber and plastic covered steel, often with factory-made clips, are the best choice of this lot.

Titanium is sometimes used on horses that need a light, very strong shoe in front. Usually, either big race horses which routinely bend aluminum (usually the near fore) or jumpers and barrel horses which must make sharp turns at speed. Titanium is easily forged or swedged into specialty configurations, but must be TIG welded. As this once very costly metal becomes cheaper, it is gaining popularity for routine use in speed horse farriery.

Historically, brass shoes were nailed on with brass nails and used in explosive atmospheres (e.g., mines).

Horseshoes have two basic configurations, open-heeled shoes and bar shoes. These basic types have an infinite number of variations which may be tailored to the individual animal. To further complicate things, a shoe may have various accouterments: clips, calks, jar-calks, screw-ins, bubbles, grabs, etc. Hopefully, these serve some purpose and enhance a particular aspect of the shoeing job.

Also under the heading of protective devices, are the various types of metal, leather and plastic pads and the many substances which are packed between pad and sole for various cushioning and medicinal purposes.

Pads come in two basic types: full or rim. These, in turn, come in two configurations: flat or wedged.

A full pad covers the entire sole; rim pads usually just the area immediately beneath the shoe. A special kind of rim pad, called a bar wedge pad, covers most of the frog area, but is open toward the toe.

Most flat pads may be from one-eighth to one-half inch thick. Wedge pads are sold in one to four degree elevations (thick at the heel, tapering to toe) which increase angulation. Pads are made from leather or synthetic (rubber or plastic) and may be soft or hard. Since leather is sensitive to moisture, it is seldom used in horses which are routinely turned out.

Most pads are placed between the shoe and foot although some special veterinary applications (i.e., a "hospital plate") are fabricated from metal and bolted to the ground surface of the shoe.

Multi-pad applications (stacks) are used in long-footed horses (Saddlebreds, Morgans, Tennessee Walkers, etc.) primarily to enhance a particular way of going and only incidentally for protection.

The substance placed between a full pad and sole is called "packing". Packing takes many forms and usually consists of a two-part application. First, some kind of medication is painted on the ground surface of the foot. This may be a proprietary preparation (e.g., Durasole, Reducine) or a "secret" mix preferred by an individual farrier. Most are iodine-based and antifungal and/or antibacterial in nature and aimed at retarding the growth of pathogens in the sole and frog. Next, the packing is placed between pad and sole, and the shoe nailed on. The basic types of packing are: oakum, sponge or foam, uncured rubber, silicone and various types of catalytic hardening soft acrylics.In addition, packing may consist of a mud-like medicated poultice.

A traction device is anything added to a shoe to enhance or increase traction. These take innumerable forms: special nails, heel calks, toe calks, grabs, jar calks, swedges, Memphis bars, etc.

Several special types of horseshoe nails are used for traction. Most are large-headed nails which protrude below the ground surface of the shoe. Mud and ice nails are two of the better-known configurations; however, an emergency traction device might consist of something as simple as a regular head nail used in a shoe punched for city heads.

Heel calks consist of downward projections of the shoe, located behind the heel nails. Their design is limited only by the farrier's imagination and the rule book for a particular breed. They are either forged (a part of the shoe), built up with foreign material (e.g., Kutrite [tm], borium), "screw-ins" or "drive-ins". Calks are usually applied singly (to the outside heels) or in pairs.

Toe calks are protuberances placed ahead of the toe nails. They are attached in the same way as heel calks with the exception that few are forged. A grab may be either forged or welded/brazed to the toe of the shoe. it usually consists of a relatively thin projection placed lengthwise, between the toenails. Technically, a grab is considered a kind of toe calk.

Jar calks are usually placed across the web (width of the ground surface) of the horseshoe. They are usually used more to send the foot in a certain direction than as a pure traction device. They are often used singly at the toe; in pairs at the heel. However, the most common usage is the familiar "mud calk" or "sticker" hind race plate which consists of a single, outside heel jar calk.

Swedge refers to a lengthwise indentation in the web of the shoe. In theory, the swedge fills with dirt which gives more traction than the parent material of the shoe alone. The ridges formed by the swedge may be the same height (rim shoes), higher outside (barrel racing shoes, Levelgrip [tm] race plates) or higher inside (polo shoes, "Argentine" race plates). The swedge is used to increase traction; the relative surface heights created by the swedge, to determine breakover.

A Memphis bar is a narrow piece of metal welded/brazed (usually, not always) across the quarters of the shoe. They are usually used singly, across the toe quarter, and on hinds. Their main purpose is traction/ breakover on long-footed horses.

Each traction device may be used in conjunction with others; thus, the usage may be customized for an individual animal to suit a specific set of circumstances. For example: a flat racer running on a sloppy track might be shod with a grab and sticker behind, and a grab and jar calks in front. The same horse, on a dry track, might run with only a grab behind and rims in front.

The final reason to shoe a horse is to effect a therapeutic change in a horse's way of going, most often to stop the horse from hitting (interfering in some way).  In reality, most attempts to modify a horse's way of going are not therapeutic; rather, they are an attempt to modify a particular gait to better meet an arbitrary (subjective) standard, usually related to a particular breed. (Non-therapeutic considerations are an important portion of pragmatic farriery.)

The specifics of this particular aspect of farriery require particular expertise and are beyond the scope of this overview. In general terms, a gait may be modified by changing the way a foot leaves the ground (breakover) or its behavior off the ground (flight path). These factors are changed by the removal/application/utilization of weight and length. A gait may also be modified by changing the timing relative to opposite members; e.g., fronts to hinds.

A horse will do whatever he does most efficiently if he is balanced, both fronts and hinds, in the two basic planes (anterior-posterior; medial-lateral) from the fetlock to the ground. Recognition, analysis and treatment of gait aberrations are some of the most difficult tasks facing the farrier.