Copper is a material that makes up some items. Copper really refers to a family of copper-alloyed metals, susceptible to corrosion but not rust and of a similar color.
The prime example is the bronze plate mail. The other main copper items are light sources: the oil lamp, magic lamp and brass lantern. There are no copper weapons.
One of the 28 randomized rings is always copper, as is one of the 27 wands.
Copper can be eaten by rock moles and xorns.
Polypiling copper objects may result in either a stone golem or a clay golem.