In networking, 'copper' always refers to the wire in your network or phone cables, which are made of this metal. Copper cable has been a standard for data transmission for many years; Cat5 and coaxial cable are two very different kinds of cable, but their conducting elements are both made of copper. Eventually, copper cable might be superseded everywhere by optical cable, which has a far greater potential capacity to transmit data, and is currently used for the main "backbone" of the Internet. However, the sheer amount of copper cable in the world (and its cost-efficiency) means it will be a very long time before it is completely obsolete.
