Online help

This is a term left over from the days when copies of paper documents were made using a sheet of carbon paper between two pieces of paper.

When you click the CC button, a menu appears showing the address book, allowing you to add names.

When you carbon copy to someone, the primary recipient is shown at the top of his email. The CC recipients are also shown as a separate list.  If you don't want the primary recipient to know who else got a copy, use BCC (blind carbon copy).