You can send email to different types of recipients:
To: The primary recipients of your message are listed in the To: field. Their names and/or email address will be visible to other recipients.
Cc: People who aren't primary recipients, but who should be advised of the message contents go in the Cc: (originally, "carbon copy") field. Their names and/or email addresses will be visible to the other recipients.
Bcc: People who should be advised of the message contents, but whose names should not appear on the message go in the Bcc: (originally "blind carbon copy") field. Their names and/or email addresses will not be visible to the other recipients.