When you put email addresses in the "To" field or in the "CC" field, everyone that received that message can see everyone else's email address. But, when you use the "BCC" field, the system hides the addresses of everyone listed in the BCC field. The "BCC" field gets its name from "Blind Carbon Copy". It was derived from the term that was originally used when typewriters were used with carbon paper to create a carbon copy of what was being made. It was a kind of "secret" copy that the addressee didn't know about.
So why would you want to use the BCC field? Here is what the United States Computer Emergency Readiness Team has to say: