Screws are fasteners inserted into material by tools, often multiple types of tools (e.g., drill, tap, screwdriver). As distinct from smooth-sided nails, they have spiral grooves on most of their length. Rather than the pure frictional resistance of a nail, which is often enhanced in wood because the nail pushes wood fibers aside, fibers which then snap against the nail shaft, a screw presents more frictional and mechanical inertia because the spiral lands and grooves provide much more surface area.
There are two broad classes of screw, self-tapping, where the screw itself cuts receiving slots into the material for the screw threads, and machine screws, for which the threaded hole is made as a separate operation. In general, self-tapping or self-cutting is used in softer materials such as wood and sheet metal, while machine screws are in hard materials. To install a threaded screw, a pilot hole is drilled with a drill bit, narrower than the eventual screw thread, and then a second tool, called a tap, usually of harder and more brittle material than the bit, enlarges the hole and cuts the grooves.
Self-tapping screws usually have a pointed tip to help them cut the hole, while screws expecting a pretapped hole have a flat tip. Some tapped holes do not go all the way through the material; they are used to fasten a second piece of work to the surface. Other machine screws go all the way through the material, and have various types of retainers attached to their tip. These retainers include nuts, washers, and various specialized clips and pins. There is a blurry line between a machine screw and a bolt (fastener); bolt tend to be larger and almost always use a fastener at the end, with exceptions such as a carriage bolt.
Driving power is applied to the screw at the head, which normally has some hard recess that receives another tool that transfers the power, generically called a screwdriver. One advantage of screws over nails is that most screws can be removed, without damage to the material, by rotating them in the opposite direction that was used to insert them. There are a great many head designs with specific applications, although the most common for hand use is a simple slot into which a flat screwdriver blade fits.