Ores

Ores are a primary crafting material for most equipment. Ores can be smelted down in a Furnace to create bars, the primary component for tools, weapons, and armor. In adition, mined ores can be placed again for use in building. Ores are usually found strewn about the underground layers of the world, however, lower tier ores may be found protruding from the surface.

Copper is the first tier of metal available to the player and can be found throughout the Underground and Cavern layers, as well as protruding from the surface in pockets. Requires 3 ore per Copper Bar.

Iron is the second tier of metal players can find. It is most prevalent throughout the Underground and Cavern layers, but pockets can also be found near the surface as well. Requires 3 ore per Iron Bar.

Found starting in the Cavern layer, Silver is stronger than Iron but weaker than Gold. Requires 4 ore per Silver Bar.

Found deeper in the Cavern layer, Gold is stronger than Silver Ore, and is the rarest ore that can be acquired in any meaningful amount by mining. Requires 4 ore per Gold Bar.

Demonite ore was the only ore that wasn't mined in previous versions. It is dropped by boss monsters, but is now also generated naturally in veins like other ores. Demonite ore is commonly found in 3-5 block veins, but is more plentiful near chasms. Combined with Shadow Scales from the Eater of Worlds it can be used to craft high tier tools, weapons, and armor. Requires 4 ore per Demonite Bar.

After destroying a Shadow Orb meteorites will randomly crash into the surface of the world, spawning a large amount of Meteorite to be mined. Meteorite burns the player without the protection of an Obsidian Skull and constantly spawns Meteor Heads when the character is near. Requires 4 ore per Meteorite Bar.

Found only in the Underworld layer of the world, Hellstone burns the player without the protection of an Obsidian Skull, Obsidian Shield, or the Obsidian Skin Potion. Hellstone is the strongest ore available for tools, weapons, and armor without activating Hardmode. Requires 1 Obsidian and 4 Hellstone per Hellstone Bar. A Hellforge must be used to smelt Hellstone bars, not a normal forge. Players can find Hellforges in the Underworld, but they can only be freed using a Meteor Hamaxe, Molten Hamaxe, or by removing the blocks below them.

Hardmode ores
After destroying a Demon Altar (which requires using a Pwnhammer or Hamdrax), Cobalt Ore, Mythril Ore, and Adamantite Ore will appear in the Underground and Cavern layers. Those ores can be found practically easily near lava in the cavern layer, in the Underworld roof and in the intersection between both.

Cobalt is the weakest of the three Hardmode ores and requires a Molten Pickaxe or better to mine. It requires 3 ore per Cobalt Bar.

Mythril is the middle tier of the Hardmode ores and requires a Cobalt Drill or better to mine. It requires 4 ore per Mythril Bar.

Adamantite is the strongest of all Hardmode ores and requires a Mythril Drill or better to mine. It requires 5 ore per Adamantite Bar. An Adamantite Forge must be used to smelt the ore into Adamantite Bars.