It's a socket wrench. Sockets are interchangeable wrench or drive bit. A driver for sockets has a square driving member that the sockets fit on. They're available in a number of standard sizes, which measure the distance across the flats of the drive. 1/4 inch drive means that it's a quarter inch across the flats, and 1/4 inch sockets fit it. 3/8 drive means 3/8 of an inch across the flats, and takes 3/8 inch sockets.