BCC is a feature of most email clients that allows you to send a message to a list of addresses without other recipients knowing about that fact. BCC addresses are concealed from recipients, including others on the BCC list.
Someone probably sent you here because you haven't been using BCC, and they would appreciate you starting to use it.
Utilizing BCC shows respect for your recipients by protecting their personal information being shared with every other recipient of the email.
When sending group emails to many people that may not know each other, it is likely at least some of those people would prefer their address be kept private.
When replying to or forwarding an email, the recipients of the original email are listed as part of your reply/forward.
This not only violates everyone's privacy, but it's just plain ugly and difficult to read through to get to the important part of your email.
Usage of the BCC feature varies depending on the email platform you are using.
Instructions are available from Microsoft by clicking here
1. Compose a new email.
2. Click the "Bcc" link to the far right of the "To" area; or press Ctrl+Shift+B on Windows or ⌘+Shift+B on a Mac.
Howto article for Yahoo Mail.
Using Outlook Web Access (OWA), you can follow these instructions.
Change the "To:" field to "Bcc:" with the dropdown list box before you enter the addresses. This is the downward arrow in a small box to the left of "To:".
Click the "bcc" link at the upper-right of the "To" text field.