Hammers have two parts--the hammer handle and hammer head.
Many hammers have handles of hickory or ash.
Use the face of the hammerhead for most work.
Some hammerheads have two identical faces, e.g. sledge hammers
Most hammerheads have only one face; the opposite side of the hammerhead is called the peen.
Machinists' hammers have round or ball peen.
Carpenters' hammers include a claw peen for pulling nails.
See also a Hammer Buying Guide at http://www.ebay.com/gds/Hammer-Buying-Guide-/10000000177630279/g.html