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, click Options on the compose toolbar. Select to show the BCC fields.
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.