Your Internet Consultant - The FAQs of Life Online
There is no functional difference between listing addresses in the To: or Cc: header lines. But from the user's point of view, it is implied that any Cc: recipients are receiving the message for informational purposes only, and no reply is desired. If anyone on the To: or Cc: list should reply to your message, the reply can (at the sender's option) go to all the recipients of your message.
The ability to send e-mail to multiple recipients is a useful tool: you can all at once (if you desire) send one message to several Internet users, an America Online account, a few fax machines, and a mailing list.