Driver bits apply torque to screws. More specifically, they are the interface that transfers torque from a tool, such as a screwdriver, t-handle, or drill, to the mating recess on top of a helically-threaded fastener. Driver bits are frequently interchangeable, come in variety of diameters and lengths, and are complemented by many accessories.
The tip of the driver bit inserts into a depression that has been machined into the top of screw head in a male tool / female fastener configuration. Female tool / male fastener combinations exist, but are less common. The rotary drive of the tool applies torque to the screw, which due to the threads machined along its shank gains traction against the workpiece.
Slotted and Phillips drives are unquestionably the most common type of screw drive, while hex socket and square are also readily available. Square and hex drives are most common for bolts. Socket drivers are often used as bolt drivers in conjunction with an extension or adapter.