Hardmode

Hardmode is a hardened version of normal world you are used to. It was introduced in 1.1 patch. You get there by defeating the Wall of Flesh in The Underworld.

Hardmode has many tougher challenges and monsters:
 * You can destroy Demon Altars with the Pwnhammer which will cause your world will be blessed with one of three new ores for each destroyed. (Cobalt Ore, Mythril Ore, and Adamantite Ore). This can be done only in world which has had Wall of Flesh defeated in it, otherwise the Pwnhammer will act like a normal hammer (and hurt the player).
 * The existing bosses will come back for revenge as The Twins, The Destroyer, and Skeletron Prime. (You can still summon and fight the old bosses)
 * Lots of existing monsters will be replaced by harder counterparts. I.e, Corruptor. (However, this does not mean they stop spawning altogether, just more rarely)
 * The Hallow will appear in the overworld.
 * The Corruption will be able to spread through almost anything. The only known thing it won't go through is The Hallow.
 * NPCs will sell new items. (The Merchant will sell the Disco Ball, the Dryad will sell Hallowed Seeds, the Arms Dealer will sell the Shotgun)