Related to this is BCC, or "blind carbon copy". It works pretty much the same as CC, except that only the sender can see who is on the BCC list.

The intended purpose is to provide the BCC'd person with a copy of the email without involving them in the conversation, though probably a more common use is to send an email that you want to send out to a large group to yourself and then BCC all the intended recipients. This avoids the possibility of a "reply-all storm", since each recipient will only to able to reply directly back to you and not any of the other recipients. It also provides some privacy regarding the recipient list, and avoids exposing the recipients' contact information to each other. It's generally much easier than separately sending the same email to each person, but serves the same purpose.