CC at the end of a letter means, literally "Carbon Copy". It has come to mean copy furnished. In the following example, the letter is being copied to three people. It is used anytime you require a copy to go to someone other than the original person.
cc: Beverly, Bill, John
NOTE: You have to be careful of how you use "cc".
If you send a copy to the wrong person, then you could be admonished for it, or jeopardize accounts.
If you unintentionally use a CC, it can cause great commission, even if the copy never really went. Everyone may think it has gone to that person. For example, confidential business plans being sent to a third party.
On the other hand, in a situation where you need the authority of someone higher up in the organization, it might be appropriate to cc that person. For example, I might need some power to close a deal with a vendor. I can cc the CEO or president of his company and my company. That usually gets things unstuck and moving.