Screws are mechanical devices used to connect and hold objects together. They consist of a head with a grooved pattern for a driver and a threaded shaft for anchorage. A user applies a torque to the head via a screw driver to insert or remove a screw. Screws may be self-tapping, which means they are able to cut their own hole or mating thread in a hole. They may also screw into an existing threaded hole. Screws are available in standard sizes and in a number of different materials, including: plastic, steel, stainless steel and brass. It is important to use a screw with a compatible chemistry to the material chassis and environment or electrolysis may occur causing corrosion.

A number of different types of screw are available, mostly in reference to the mating grooved pattern for the screwdriver. There are a large number of patterns including: flathead, Pozidriv, Phillips, square (Robertson), hex, star and cross-recess. Manufacturers can utilize odd types to try to prevent users from undoing screws for safety concerns and to prevent voiding warranties.

Most screws can be both inserted and removed, but there are one-way screw types that allow easy insertion but have a pattern that makes removal very difficult (one-way clutch screws). Torque sets are a special screw head that have offset cross arms to prevent normal Phillips or Pozidriv screwdrivers being used – forcing tools with proper torque settings to hopefully be used instead. Over tightening can cause stress and damage to the screw. Screws are sometimes secured with split rings or similar washers. Locktite is frequently used to provide a glued seal that helps prevent the screw undoing through vibration. A thumbscrew is a special type of screw with a wide head that has knurled sides to allow for a person to tighten or untighten without a screwdriver.