Additionally to the responses you have received so far, you would include recipients in cc to let them know that they need to be in the know, but that you are not expecting them to respond. eg.if you need to inform your colleague that you have completed part of the task assigned to you and your colleague can take it forward, you would mark your colleague on the mail in the To: tab. Should you wish to keep your boss in the loop, you would mark a cc to him. You would not expect him to respond on this routine matter.
You would bcc people, if you are mailing to people who are strangers to each other and who do not need to relate to each other either in response to the mail or ever, henceforth. In a more sinister way, you would bcc a person who you would want should be in the know but secretively, so that no one, either marked or copied in the mail will know of your intentions.