Socket screws have an allen wrench drive.
Socket caps have a small cylindrical head with tall vertical sides. Allen (hex socket) drive is a six-sided recess for use with an Allen wrench (hex key).
Bits for Allen drive screws (a.k.a. socket products).
Screws with an Allen wrench drive.
Screws with no head and machine threads for use in a tapped hole.
Screws with an unthreaded shoulder that is larger in diameter than the threads. Also known as stripper bolts.