When I first read your topic title, I read "What's BBC". I see that the question has been answered already. I typically use BCC for sending email to a list without revealing to anyone who else is on that list.
BCC can prove dangerous in some instances. This one time, I BCC’d my boss on an email that I sent to a co-worker. The boss didn’t realize that he was BCC’d and replied back (to all). Obviously the co-worker wasn’t very happy but I kinda laughed it off.
I have also replied to an email I was BCC’d on and was supposed to be a silent observer of the issue we were facing at that time. I just didn’t realize I was BCC’d on it.
I try to avoid BCCs on outbound but can’t control the ones that come to me.