The Phillips drive has an x-shaped slot that accepts a Phillips driver and is designed to allow the driver to slip out of the head to help prevent over-tightening and damage to the thread or fastener.
Flat head is cone shaped for use in countersunk holes and has a flat top that fits flush with the mated surface.
Machine screws are threaded fasteners that are typically used with nuts or drilled and tapped (threaded) holes.