A horseshoe is U-shaped piece of iron, rubber, plastic, rawhide or a laminate of these, nailed or glued to a horse's hoof--very like a shoe. Horseshoes, kept as a sort of talisman and not on a horse, are said to bring luck.
Since the early history of man's domestication and use of horses, three factors have contributed to the need for the bottoms of the horse's feet (hooves) to have additional protection over and above their natural hardness.
First, the added weight/stress of a humans, cart or wagon traces or of pack loads; second, the fact that in domestication, the customary amount of ground covered by a horse on a daily basis is greatly curtailed and third, the fact that live food eaten in the wild is nutritionally superior for the purpose of building strong hooves. In nature, the horse walks and grazes continuously over a wide variety of surfaces. The consequence of this nonstop travel on the horse's feet is to keep them worn to a small, smooth, even and hard state. The continual stimulation and irritation of the sole of the foot keeps it thick and hard, much like a callus. Live grasses, weeds and shrubs are high in nutrients such as beta carotene. Man-raised feeds lose a high proportion of their carotene within hours of harvesting, as so do not provide this vital ingredient to the horse. The hoof is made of horn, much as the human fingernail, and grows hard, tough and flexible only with optimal nutrition.
In captivity, absent the natural conditioning factors present in the wild, the feet of horses grow overly large, long, fragile and soft. Hence, protection from rocks, pebbles and hard, uneven surfaces is lacking. Cracks in overgrown and overly brittle hoof walls are a constant danger, as is bruising of the soft tissues within the foot because of inadequately thick and hard sole material. Man has sought to remedy this lack with suppplemental support and armor, beginning in the earliest days with rawhide boots which could be tied onto the hoof. Since that time, man developed metal shoes nailed to the rim of the sole with nails which find a purchase in the hoof wall. In modern times, these nails are applied so as to enter the sole of the foot very near the edge, and at an angle which causes them to protrude through the hoof wall, where they are then bent over, cut off and "clinched" to hold in the hoof wall.
Advances in technology and materials have led to shoes which can be glued to the bottom of the foot, and which are composed of tough but yielding materials. This cushions ground impacts without adding rigidity or excessive weight to the foot and without requiring nail holes. Typically such applications respond to special needs or medical problems of a given animal, and are not routine. Iron is still favored as the most desired material for horse shoes, because the rigidity which it provides protects the hoof against certain types of injury (such as heel shear) which other materials do not protect against.