"Flails, like maces, are primarily bludgeoning weapons. While harder to wield than traditional maces and clubs, these spiked steel weights suspended from heavy chains allow a Crusader to inflict heavenly fury upon any foes foolish enough to stand in the way."— Game Guide(src)
Flails are a Crusader-only variant of maces in Diablo III (prior to the third game, they also existed, but were classified as maces). They are significantly faster than regular maces. While normal maces are using a solid handle, each flail uses a blunt or sharped head on a flexible chain.