The type of wrench used to turn screw or bolt heads designed with a hexagonal socket (recess) is called an Allen wrench or hex key. An Allen wrench is a simple hand tool consisting of a L-shaped or T-shaped bar with a hexagonal-shaped key at the end. The hex key matches the size of the hexagonal socket on the screw or bolt head, allowing for a secure fit and efficient turning. Allen wrenches are commonly used in various applications, including furniture assembly, automotive repairs, and machinery maintenance.