What is the meaning of CC & BCC in Email formats?

The CC Header

The CC header really works just like the To header; it indicates that the message is sent to the addresses in it, too. The difference is that if you're in the To header it means the message is aimed specifically at you, while if you're in the CC header it just means that you're being copied on a message aimed at somebody else. It's more of a social distinction than a technical one. "CC", by the way, stands for a piece of archaic, pre-computer technology: Carbon Copy.

The BCC Header

The BCC header works like the CC header, indicating that the message is to be copied to all the addresses on this list, except that this is a blind copy: the recipients aren't supposed to know who else the message is being copied to. This means that the server is supposed to strip out the BCC header before delivering each copy of the message. Only the sender gets to see the contents of this header. But remember that nothing is truly private or secure on the Internet unless it's sent end-to-end by encrypted protocols; just because you cause the messages to omit mention of the other recipients doesn't mean that somebody, somewhere, won't purposely or accidentally discover who you sent the message to. If there are bad addresses on the list, for instance, the message might get bounced to the server's postmaster.