If you’re sending a message to several people and want to hide each recipient’s email address, you can use the blind carbon copy feature. When you’re composing a message, just click Add Bcc and enter the email addresses that you don’t want to display to the recipients.
How you know when you’ve been Bcc’d
When someone Bcc’s your address on an email that was sent from another Gmail or Google Apps user, Gmail will show your address in the “Bcc” line to let you know that you’ve received this email by way of Bcc. This way, you won’t be confused about why you’re receiving a message when your address isn’t in the “To” or “Cc” line.
Similarly, if an email sent by another Gmail or Google Apps user has reached you because you’re a member of an email group that was Bcc’d, that group’s address will show in the “Bcc” line.
Only your address (or the email group you have subscribed to) will be shown in the “Bcc” line; if any other email addresses were Bcc’d as well, you won’t be able to see them.
If you’re Bcc’d on an email that was sent using another email program or service, Gmail may not be able to display this information.
How the people that you send email to will know that they’ve been Bcc’d
When you include Bcc recipients on an email that you send using Gmail, we add a specific header for each individual Bcc recipient to let them know that they’re receiving the email because they are Bcc’d on it.
Each Bcc recipient only sees their own address in the message that they receive.
If the recipients use Gmail or Google Apps to read their mail, they’ll see this header. Otherwise, depending on how their email systems handle the header, recipients may or may not see the information. [source]