Nails are mechanical fasteners that are driven into materials such as wood with a hammer or nail gun. They are made of metal or plastic materials and differ in terms of head style. Common types of nails include brad nails, cap nails, double-headed nails, finishing nails, masonry nails, and roofing nails. A brad nail is a flat nail with a head on one side. A cap nail has a square or round head. A double-headed nail has a collar that is located away from the head. A finishing nail is designed for applications where the nail head must be hidden from view. A masonry nail has a single head and is usually made of hardened steel. A roofing nail has a relatively large head and is designed to resist weathering. Nails with other head styles are also available.