Stands for "Blind Carbon Copy." When you send an e-mail to only one person, you type the recipient's address in the "To:" field. When you send a message to more than one person, you have the option to enter addresses in the "Cc:" and "Bcc:" fields. "Cc" stands for "Carbon Copy," while "Bcc" stands for "Blind Carbon Copy."
A carbon copy, or "Cc'd" message is an e-mail that is copied to one or more recipients. Both the main recipient (whose address is in the "To:" field) and the Cc'd recipients can see all the addresses the message was sent to. When a message is blind carbon copied, neither the main recipient nor the Bcc'd recipients can see the addresses in the "Bcc:" field.
Blind carbon copying is a useful way to let others see an e-mail you sent without the main recipient knowing. It is faster than sending the original message and then forwarding the sent message to the other recipients. It is also good netiquette to use Bcc when copying a message to many people. This prevents the e-mail addresses from being captured by someone in the list who might use them for spamming purposes. However, if it is important that each recipient knows who your message was sent to, use carbon copy (Cc) instead.