Flat head socket cap screws have a 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.  The flat head allows this socket cap screw to sit flush with the surface.  Black oxide is the most popular finish, but button head socket cap screws also come in zinc and stainless steel.