Shapes of bolt and screw head

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Screws come with all kinds of different shaped heads. The different shapes are generally used for different applications. Here's a quick list of the most common head shapes.

Flathead (or countersink) screws have, not surprisingly, a head with a flat top. The head also has a tapered base that allows it to be driven flush with the surface when installed in a countersunk screw hole. This screw is most commonly used in furniture making where you want to be able to hide the screw head.
Pan head and oval head screws both have rounded sides, but the pan head is flat on top while an oval head has a rounded top and a tapered base that can be counter sunk. Both are often used for sheet metal work.
Round head screws are shaped almost like a half circle with a flat bottom. Even after it's been driven in, the head sits "proud" or above the surface. These screws are used to fasten material that is too thin to allow counter sinking and are often used in combination with a washer.

Pan head
A low disc with chamfered outer edge
Button or dome head
Cylindrical with a rounded top
Round head
Dome-shaped, commonly used for machine screws
Truss head
Lower-profile dome designed to prevent tampering
Flat head or countersunk
Conical, with flat outer face and tapering inner face allowing it to sink into the material
Oval or raised head
Countersunk with a rounded top
Bugle head
Similar to countersunk, but there is a smooth progression from the shank to the angle of the head, similar to the bell of a bugle
Cheese head
Disc with cylindrical outer edge, height approximately half the head diameter
Fillister head
Cylindrical, but with a slightly convex top surface. Height to diameter ratio is larger than cheese head.
Socket head
Cylindrical, relatively high, with different types of sockets (hex, square, torx, etc.)
Mirror screw head
Countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors
Headless (set or grub screw)
Has either a socket or slot in one end for driving
Square head
A 4 sided head used for high torque driving with a wrench.
Anchor head
An adaptation of the standard carriage bolt design. Possesses a truss head on a square neck, which resists rotation when located or driven into place.
Binding head

A type of head that is similar to the pan head but much thicker with a deeper slot.
Flange hex head
Similar to hex washer with the exception that the top of the washer-flange shall be conical or slightly rounded within the periphery of the flange diameter. The contour of the flange edges shall be optional provided minimum flange thickness is maintained.
Flat Fillister head
A type of head that is similar to the cheese head but with a flat top for better appearance.
Hex wahsher head
Has an indented top surface, six flat sides with a flat washer which projects beyond the sides and provides a flat bearing surface. The washer and hex are formed together as one piece.
Indented hex head
Have an indented top surface, six flat sides, and a flat bearing surface.
Trimmed hex head
This is the standard type of wrench - applied hexagon head, characterized by clean, sharp corners trimmed to close tolerances.
Washer head
A countersunk head with a flat top surface and a cone-shaped bearing surface. The wafer's 70° conical under head area does not extend to the outer edge of the head, providing a bearing surface of 16° around the circumference of the under head.