From McMaster-Carr..........
About Counterbores
Counterboring— Enlarge the top portion of an existing hole, forming a cylindrical-shaped opening with a flat bottom that acts as a seat for the head of a screw or wood plug.
Shallow Counterboring— Bore a shallow spot (1/8" or less) around a hole so that the head of the screw, bolt, or nut is slightly raised above the surface of the material (also known as spot facing).
Pilots— A pilot guides the counterbore as it penetrates the material, keeping it centered over the drilled screw hole and ensuring a level seat for the screw head. Counterbores with built-in pilots have a counterbore and pilot made from one piece of steel. Changeable-pilot counterbores have a removeable pilot, allowing you to use various pilot diameters in a single tool.
About Countersinks
Countersinks form a cone-shaped opening at the top of a hole, which acts as a seat for the head of a screw or rivet. They can also be used for chamferring, deburring, and creating openings for holding material between centers.
Flutes
Fast Cut — Have one flute and won't vibrate at high speeds. Also great for cutting smaller holes.
General Purpose — Have two, three, or four flutes. Tool life increases with more flutes because the cutting load is distributed over more edges. However, fewer flutes provide better chip clearance, which is a consideration when machining stringy materials like plastic.
Smooth Finish — Have six flutes. These remove more material per revolution and have a longer tool life than other countersinks.
Body Diameter and Countersink Angle
The countersink body diameter must be equal to or larger than the head diameter of the screw, center, or rivet being countersunk. Use a pilot hole that's larger than 10% of the countersink body diameter.
60° Countersink Angle: For holding workpieces between the centers on a lathe.
82° Countersink Angle: For flat- and oval-head inch screws.
90° Countersink Angle: For flat- and oval-head metric screws. They're also great for chamfering.
100° Countersink Angle: For rivets.
120° Countersink Angle: For rivets.
............or in a game of Battleship, it is sinking the other guys ship.....