Torx® Bits

TORX, developed by Camcar LLC of Acument Global Technologies (formerly Camcar Textron), is the trademark for a type of screw head characterized by a 6-point star-shaped pattern (in the same way that slotted heads, Phillips, hex socket, and Robertson have linear, cruciform, hexagonal, and square tips, respectively). People unfamiliar with the trademark generally use the term star, as in star screwdriver or star bits. The generic name is hexalobular internal driving feature and is standardised by the International Organization for Standardization as ISO 10664. TORX is frequently styled Torx despite the official all-caps trademark styling.

By design, TORX head screws resist cam-out better than Phillips head or slot head (flathead) screws. Where Phillips heads were designed to cause the driver to cam out, to prevent over-tightening, TORX heads were designed to prevent it. The reason for this was the development of better torque-limiting automatic screwdrivers for use in factories. Rather than relying on the tool slipping out of the screw head when a torque level is reached, and thereby risking damage to the driver tip, screw head and workpiece, the drivers were designed to achieve a desired torque consistently. Camcar LLC claims this can increase tool bit life by ten times or more.

- Wikipedia

Here is their main Torx webpage.

This table presents the most common sizes.