A mace is a club with an extra heavy weight at the end. Unlike a hammer which must be aligned with its target, a mace can strike in any direction. They are often confused with flails, but those are a clearly distinct class. While a flail has its head(s) attached to the handle via (a) short chain(s), a mace head sits directly on top of the handle.
Stone age maces had wooden handles and - of course - a stone head. Later maces were equipped with copper, bronze or iron. Sometimes the handle was made of metal too, rather than wood. Most maces had simple round heads, others angular, to concentrate the force of the blow into a smaller striking surface, increasing pressure. A popular design was the knobbed mace, with a head that is studded by small knobs. Another common type was the flanged mace, which consists of four or six blunt blades, 'flanges', at regular intervals around the head. Flanged maces did not become popular in Europe until the 12th century CE, when armor got thicker and heavier.
Most footman's maces tended to be short, 0.7 - 0.9 meters in length on average. Cavalry maces were longer to provide more reach, but lighter. There also existed two-handed maces that were both long and heavy.
Maces are easy to make and cheap. Like other clubbing and smashing weapons they are rather slow, but the impact of a mace is heavy and can cause concussion damage even when the opponent is protected by good armor. This explains their popularity in battles against heavily armored knights. Medieval priests also wielded them because they shed little blood, though they can be just as deadly as cutting and stabbing weapons.