@majimaji usiwe majimaji. You only need to google some to these things!
http://en.wikipedia.org/wiki/Blind_carbon_copy
'In the context of e-mail,blind carbon copy (abbreviated Bcc:) refers to the practice of sending a message to multiple recipients in such a way that conceals individual email addresses from the complete list of recipients.
While now associated almost exclusively with e-mail and other electronic messaging,the term originates with typewritten documents. In a now-rare practice,a typist produces multiple copies of a document by alternating one or more layers of carbon paper between sheets of blank paper. When the typewriter letter strikes the paper,the carbon transfers to the paper,producing a copy. In some circumstances,the typist must ensure that multiple recipients of such a document not see the names of other recipients. To achieve this,the typist can:
Add the names in a second step to each copy,without carbon paper
Set the ribbon to not strike the paper,which leaves names off the top copy (but may leave letter impressions on the paper)
To specify recipients,an e-mail message may contain addresses in any of these three fields:
To: Primary recipients
Cc: Carbon copy to secondary recipients—other interested parties
Bcc: Blind carbon copy to recipients who receive the message without others,including the To: and Cc: recipients,seeing who else received it
It is common practice to use the Bcc: field when addressing a very long list of recipients,or a list of recipients that should not (necessarily) know each other,e.g. in mailing lists.'