Basic Information
A maul is, essentially, a big hammer - very similar to a sledgehammer and bordering on being an improvised weapon. It's also what a lot of fantasy artists wrongly assume a warhammer to look like. This is a bludgeoning weapon par excellence, and if rather ungainly, can deliver plenty of blunt force.
These probably first made it onto the battlefield as engineer tools, improvised as weapons - there is some indication, for example, that they were used on dismounted French knights at Agincourt, having previously been used to help lay the defensive stake barrier. There is further evidence of mauls having been deliberately issued as weapons and this - and the fantasy confusion - is why they get a "weapon" entry of their own.
Note that most fantasy artists - and many miniatures producers - show the head of a maul far too big as well as misidentifying it. Even the most rudimentary handling of a modern sledge will prove that a much smaller load would be needed for a practicable weapon.
For anyone interested in Norse mythology, Mjolnir the hammer of Thor probably qualifies as one of these rather than a warhammer.