Driver bits are instruments which apply torque to a screw. A screw creates a high-friction assembly by shearing the workpiece(s) with a set of sharp, helical threads that run the length of the screw shank. The screw must be rotated so the threads can guide the screw shaft into the substrate, and this is most commonly accomplished by outfitting a screwdriver or drill with driver bits. Driver bits are frequently merchandised in complementary sets.
The head of the screw incorporates a recessed pattern to allow the transmission of torque from the tool/driver to the screw shaft. This pattern mates with a specific driver style, and an extensive number of screw patterns are available. These patterns are often sized according to imperial or metric conventions, or are provided a numeral denomination which is relative to the drive size but not its exact measurement (e.g. #2, #4, #6). Furthermore, tamperproof drive bits are used to torque security screws—a screw with a pattern that is innaccessible by commonplace driver bits. There are also driver bit extensions to lengthen the distance between the driving tool and the screw.
Driver types sometimes have multiple names, and include: flat/slot; Phillips; cross (not to be confused with Phillips); square/Robertson; hex/Allen®; hexlobe/Torx®; star; tri-wing; torq; triangle; spanner; clutch/one-way; bristol, triple-square; pentalobe; and all tamperproof/security variations of the aforementioned drives.
Images credits: Direct Industry; Wikimedia