Horseshoes are things worn by horses to protect their hooves (feet) when they walk - like a shoe. Most horseshoes are made of iron or steel. Some are aluminum, rubber, plastic, rawhide or a combination of materials.
A farrier is someone who makes horseshoes and nails or glues them on the horse's hooves. Using nails does not hurt the horse, as the outer part of the hoof cannot sense pain.