Stands for "Carbon Copy." The term comes from carbon copying, in which a piece of carbon paper copies writing from one paper to another (often used when filling out forms). However, the term is now commonly used in reference to e-mail. When you send an e-mail message, you typically type the recipient's address in the "To:" field. If you want to send the message to one or more other recipients, you can use the "Cc:" field to add additional addresses. This will send the e-mail to the address in the "To:" field and to each address listed in the "Cc:" field as well.
The "Cc:" option is often used in business communications when a message is intended for one person, but is relevant to other people as well. For example, a retail employee may e-mail another employee saying he can work for her on a certain day. He might include his manager's and assistant manager's e-mail addresses in the "Cc:" field to let them know he is taking the work shift. Similarly, a team member working on a product design may e-mail his boss with the latest design revisions and may "Cc:" the other members of his team to let them know the e-mail has been sent.
"CCing" (yes, it can also be used as a verb) is a quick way to let other people in on your e-mail communications. It is efficient because you don't have to send separate messages to each individual address. However, remember that When you Cc an e-mail, all the recipients can see the other addresses the message was sent to. If you want to hide the additional addresses, use Blind Carbon Copy (Bcc) instead.