A torque wrench is a tool used to apply a specific torque to a fastener such as a nut, bolt, or lag screw. It is usually in the form of a socket wrench with an indicating scale, or an internal mechanism which will indicate (as by 'clicking', a specific movement of the tool handle in relation to the tool head) when a specified (adjustable) torque value has been reached during application.
A torque wrench is used where the tightness of screws and bolts is a crucial parameter of assembly or adjustment. It allows the operator to set the torque applied to the fastener to meet the specification for a particular application. This permits proper tension and loading of all parts.
Torque screwdrivers and torque wrenches have similar purposes and may have similar mechanisms.