A socket wrench is a wrench with interchangeable heads called sockets that attach to a fitting on the wrench, allowing it to turn different sized bolts and other fasteners. The most common form is a hand tool popularly called a ratchet consisting of a handle with a ratcheting mechanism built in, so it can be turned using a back-and-forth motion in cramped spaces. A socket has a cup-shaped fitting with a recess that grips the head of a bolt. The socket snaps onto a male fitting on the handle. The handle supplies the mechanical advantage to provide the torque to turn the socket. The wrench usually comes in a socket set with many sockets to fit the heads of different-sized bolts and other fasteners. The advantage of a socket wrench is that, instead of a separate wrench for each of the many different bolt heads used in modern machinery, only a separate socket is needed, saving space.

There is also a power tool version, in which a socket set is used with an impact wrench. The wrench is usually powered pneumatically, although electric versions are not uncommon. Hydraulic motor versions are rare outside of heavy industry. The sockets for impact duty (called impact sockets) are made with thicker walls and tempered to a lower hardness so as not to shatter under the impacts. They are typically finished in black oxide rather than the chrome plating typical of the hand-tool variety. Hand-tool sockets (ie. non-impact sockets) should never be used with impact wrenches as they will shatter.