There are many different types of screws -- machine screws normally do not have a point and are screwed into existing threaded holes or nuts.  Wood and sheet metal screws normally have a sharp point for penetrating the material where there is no hole.  Screws may have flat, square, round, or countersunk heads, or they may not have heads at all.  Machine screws are categorized by threads-per-inch, which differs according to the diameter of the shank of the screw.  For instance, a 1/4" diameter screw normally has 20 threads per inch.  A 1/2" diameter screw normally has 16 threads per inch.