A hammer is composed of a shaft and a head, assembled in a 'T' formation.
The shaft (generally of wood) fits between the user's hand and the hammer's head. It is what you grab to use the hammer. The longer the shaft, the more force can be exterted by the head, but at the same time, the slower the hammer becomes to use.
The head (generally made of metal or (in primitive societies) of stone) is what does the striking -- the striking surface with generally be flat or slightly convex. There may be a second striking surface on the other side of the 'T', of the same size and shape, or designed differently, for a different striking effect.