The abbreviations Cc: and Bcc: stand for carbon copy and blind carbon copy, respectively.
Cc: lets you send a copy of a message to someone who's interested, but is not the primary recipient. All Cc'ed recipients see the entire list of addressees when they read the message.
Bcc: lets you send a copy of a mail message to someone without their address appearing in the copies of the message sent to other recipients.
In the compose window, select Show BCC.
Click the To:, Cc: or Bcc: button that appears next to the address fields to bring up the address selection dialog if desired. Otherwise, enter your addresses directly in to the desired fields.
You can send a mail message without any addresses in the To: field, as long as there is at least one address in either the Cc: or Bcc: fields.