Nails come in two major categories — finish nails, also called brads, and common nails. Finish nails have small heads that are only slightly thicker than the shaft of the nail, which makes them nearly invisible when driven in. These are used for floorboards, molding, and picture hooks. Common nails have a larger round head and are used for structural work that's usually hidden — from building decks to the rough structure underlying a house. Nails are held in place by friction created by compressed wood fibers; they hold tight but can be pulled out by force.