Hooks come in various sizes, shapes and the material they're made of.  There are also the small steel hooks that are used with very fine cotton yarns.  Aluminum and steel hooks sometimes are manufactured with plastic handles for a better grip (called "soft touch" handles).  These plastic handles also put less pressure on the fingers.  One thing to be cautious about is that there appear to be no standard hook sizes among manufacturers.  The points and throats of different brands of hooks come in different shapes and these shapes affect the size of stitch they produce.

Parts of a Hook:  a crochet hook is made up of four parts - point, throat, thumb rest, and shank.  The point goes into the stitch on the crocheted fabric; the throat catches the yarn.  Note that the throat has to be sized accordingly in proportion to the yarn being used. 

The shank holds the loops that you're working with and is the part of the hook that determines stitch size.  Finally, the thumb rest is an area where you rest your thumb to help you turn the hook easily while working.

Going from smallest to largest, aluminum or plastic hooks go from size B to Q - 2.50 mm to 15.00 mm (US), and size 14 to 2 - 2.00 mm to 7.00 mm (British).   One good thing to bear in mind is that the size of your yarn dictates your hook size.