A sledgehammer is a tool with a large, flat head attached to a lever (or handle). The head is typically made of metal. The sledgehammer can apply more force than other hammers, due to its large size. Along with the mallet, it shares the ability to distribute force over a wide area.
The Sledgehammer is your second go-to melee weapon in DayZ, after the Firefighter Axe. It will take out any normal zombie in 2 hits, and a military zombie in 2-3 hits. It also has a shorter handle than the axe, and is therefor easier to conceal on your back.