In Eudora and perhaps other email facilities, you'll see the abbreviations "Fcc" and "Bcc". "Fcc" is the same as "cc" or carbon copy - that is, send a copy of the message to the address you fill in. "Bcc" stands for "Blind carbon copy" which says: send a copy to another address...but in this case, don't indicate to the Fcc recipient that you also sent this copy.
Since today the term courtesy copy is sometimes used instead of carbon copy, Bcc can also stand for blind courtesy copy. However, it is sometimes considered impolite to use Bcc because not all recipients may be aware of everyone that will receive the message.
There are legitimate uses of Bcc. For example, it's one means of sending out a message to a large group without displaying the email addresses of all. That capacity is useful for a number of purposes, including maintaining privacy and avoiding large blocks of text devoted to addresses in the body of an email message.