A sledgehammer is a large hammer used for pounding sledges. What's a sledge, you may ask? WHO THE F**K KNOWS?! Its just a square lump of metal on a wooden stick used for pounding metal and whacking in railroad ties. It's like the older more well-endowed brother to the mallet.
Some folks elect to include a sledgehammer as part of their arsenal of weaponry. While the blunt force trauma executed by a deftly handled sledgehammer may be effective, it is a clumsy and unwieldy item and not really recommended for reigning personal wrath upon one who has wronged you.