Copper is a very important resource. It is mostly used in early stages of the game as Copper Cables and in building IC2 EU generation devices. To double the output, macerate 1 Copper Ore to yield 2 Copper Dust, then by smelting the dusts, you can get 2 Copper Ingots instead of the normal 1. You can also combine Copper and Tin dusts and make bronze dust, which can then be smelted into bronze ingots.
The best place to fine copper ore is around level 48.
See Copper Ingot.
Copper can be found in layers 36 and higher. It is most commonly found in between layers 40 and 55.