The war hammer is a medieval weapon and a common item of equipment in most Fantasy Role-Playing games.
The war hammer is a descendant of the lucern hammer, designed to be used by knights on horseback. The hammer is usually made completely from steel with a squared head, and a handle of about 18 inches in length. These are often highly decorated, as are most weapons used by the upper classes.
As far as RPGs and fiction go, the war hammer is the preferred weapon of the Dwarven race. Mainly because of the social and religous significance that the hammer enjoys in Dwarven society. This is also a common clerical weapon in the various AD&D worlds.