A farrier is a hoof-care specialist who trims, balances, and shoes horses feet. The work joins blacksmithing to practical anatomy: the farrier reads how a horse stands and moves, trims the hoof to a correct angle, and fits or forges shoes when needed. Regular visits, usually every six to eight weeks, keep the hoof sound and head off lameness.
See also: the structure a farrier works on, angle the farrier trims to match
Further reading: Farrier on Wikipedia; hoof care of horses from the Merck Veterinary Manual.