BCC (Blind Carbon Copy) or Blind copy is a way of sending email without enclosing the addresses the email was sent to. The recipient addresses in the BCC field will recieve the email, but will not be shown for those who recieve the email. If all the recipient addresses are added in the BCC field, the To field will show "undisclosed-recipients" for those who recieve the email.
Using BCC is a way of avoiding distribution of email addresses. If you send an email to several recipient addresses by using the To- and/or CC field, all those who recieve the email be able to see the other addresses the email was sent to, which can be preferred sometimes. On the other hand, if there is no special reason for enclosing the recipient addresses in the email, BCC is a way of avoiding this.
To send a BCC you write the recipient addresses you want to send a BCC to in the BCC field. If you want to use the BCC field for all your recipients some email clients might object to not having a recipient in the To field. To avoid this problem you can put your own email address in the To field, this way you get a copy yourself which you can delete if you don't need it or keep it for archiving purposes.
Here is a small overview of how to use BCC with some commonly used email clients
Outlook Express doesn't show the BCC field per default, but you can easily display it by pressing View in the meny and check off All headers.
In Mozilla Thunderbird you choose what way to send the email for each field. To send a BCC you choose BCC from the pulldown menu in the type field, as shown in the image below
When using Webmail the BCC field is shown by default.
In Pine the BCC field is not shown by default, but by pressing Ctrl+R which shows "Rich Headers" the BCC field appears among some other choices, as shown below