Hard Mode is an aptly named world-specific mode that is unlocked by defeating the Wall of Flesh. The Corruption or The Crimson spread more rapidly in Hard Mode, and a new biome known as The Hallow is created. New monsters, bosses, and items are available in Hard Mode as well. This mode cannot be turned off - defeating the Wall of Flesh will permanently set that world to Hard Mode, unless one uses cheats. The only way to convert a world back to Pre-Hardmode is through third party editing.