Cc: stands for "carbon copy." Anyone listed in the Cc: field of a message receives a copy of that message when you send it. All other recipients of that message can see that the person you designated as a Cc: recipient received a copy of the message.
Bcc: stands for "blind carbon copy." This is similar to the Cc: feature, except that Bcc: recipients are invisible to all the other recipients of the message (including other Bcc: recipients). For example, if you send a message To: johndoe@yahoo.com and Bcc: janedoe@yahoo.com, then johndoe sees himself as the message's only recipient. Janedoe, on the other hand, is "in the know"—she can see that you sent the message To: johndoe, and that you blind-copied her. To add an entry in the Bcc: field, click the "Show BCC" link to the right of the "To:" field.