Socket head cap screws have a round cap with hex socket (Allen) drive.  The screws are tightened with a hex key or allen wrench.  Commonly found in machine assembly where you do not have enough space to tighten with a wrench or socket.  Socket head cap screws can withstand more torque than a button head or flat head socket cap screw because of the deeper drive.  Black oxide is the most popular finish, but socket head cap screws also come in zinc and stainless steel.