Copper Ore is the most common metal in Terraria. It is found in abundance on and just below the surface, as well as in most caves. Three copper ore can be smelted to make a Copper Bar using a Furnace. With this, the third tier of armor can be crafted. Copper Ore and Stone can also be combined to make a Copper Brick.
Since v1.2, there is a chance that the world will have all of its Copper replaced with Tin Ore during world generation. This will only allow the player to craft Tin Bars instead of Copper Bars. Those two ore types may look different, but their functionality is the same.