The round head screws have a curved head and a flat lower surface. When the screw is fully inserted, the head will still protrude and will, therefore, be easily visible. The advantage of using the round heads is that you don't have to countersink holes. Round head screws are mainly used in sand rails.
Countersunk screws are screw types that have a long shank and tapering ends. The head of this type of screw could be of two types: countersunk slotted or countersunk cross head/pozidriv head. The head of the first type has a one slot running across the head, while the head of the countersunk cross head has a cross-shaped notch. Countersunk slotted is mainly used for wooden items such as in door hinges and locks, while the countersunk cross head is used in fixing brackets of machines. The cross-head screw has an advantage over the slotted screw, because the cross-head type bearing a cross-shaped notch prevents a screwdriver from slipping.
The flat-head screws are those whose heads lie flush with the item in which they are being inserted. When using flat-head screws, countersunk holes are a must. This category of screws is mainly made of steel plating with zinc. making them corrosion-free. Flat-head screws are mainly used in marine industry since the zinc coating resists corrosion caused by water.
The truss heads, like the round heads, feature that which protrude well off the surface of the material they are inserted into. The main difference between the truss head and the round head is that the heads of the truss heads are generally larger. The truss heads are used for mounting servos.