Machine screws come in a variety of head styles.  A particular head style may be chosen for decorative purposes, functionality, or a combination of both.  Some head styles can countersink into the mounting surface while others will sit atop the surface.  They may be driven in by a Phillips bit, a Slotted bit, or Hex nut driver.

Countersunk Heads

Countersunk head screws are designed to sink down into the surface they are being installed in.  This provides a smooth flush surface once installed with the exception of the Oval Head which still maintains a very low profile on the mounting surface.

Rounded Heads

The rounded head machine screws all have round outside edges with varying diameters and profiles (height above surface mounting area.)  The Fillister heads are the tallest and the Truss heads have the lowest profiles while the Pan and Round heads are typically more dome shaped.

Hex Heads

The hex heads are six sided hexagon shaped heads that sit atop the mounting surface.  They have sharper edges than the round and countersunk heads but can be installed with more torque.