What is Cc and Bcc in the Mail

Carbon Copy and Blind Carbon Copy

CC means send to a user but they are not the primary recipient. Others will see the address but know that you are just keeping that person informed.

BCC means send to a user and only show that user's email address. If I send to a group of people I use BCC for all of them so I keep everybody's address private.

You can have all or just some in each recipient field.

https://en.wikipedia.org/wiki/Carbon_copy

In the days of typewriters and impact printers (dot matrix, daisy wheel) there used to be a way to print two copies simultaneously on two paper sheets stacked with a thin sheet of carbon ink in between them.

There are also some pressure sensitive carbon-less forms in duplicate or triplicate sheets, when you write on them in an ink pen it transfers the writing to the sheets underneath and is not as messy as a carbon ink sheet, like in duplicate checks, you rip out the check and keep a not-negotiable copy of it.

When someone hits Reply-All, either inadvertently or intentionally, everyone in To: and Cc: is included, but none of the Bcc: names are included in Reply-All. So clever use of the Bcc: is also a way to avoid a maelstrom of a mail storm when sending to a large group and everyone wants to share their own opinion or ask to be removed from the resulting chain mail, or instruct everyone on the Reply-All chain on the etiquette of using Reply-All. etc, etc, ad nauseum...