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 3 new ores for each destroyed. (Cobalt Ore, Mythril Ore, and Adamantite Ore).
 * The existing bosses will come back for revenge as Skeletron Prime and The Destroyer as well as The Twins.
 * Lots of existing monsters will be replaced by harder counterparts. I.e, Corruptor.
 * The Hallow will appear in the overworld.
 * NPCs will sell new items. (The Merchant will sell the Disco Ball, the Dryad will sell Hallowed Seeds.)
 * The Corruption will be able to spread through almost anything. The only known thing it won't go through is The Hallow.