Socket screws have an allen wrench drive.
Flat heads are countersunk (cone shaped under the head) with a flat top. Allen (hex socket) drive is a six-sided recess for use with an Allen wrench (hex key).
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.