CC: Carbon Copy, sends a copy of the email to everyone you check. Also, everyone can see who got cc's, so everyone knows who is in on the email.
BCC: Blind Carbon copy, means that someone gets a copy, but is not listed on any other copies of the email, so no one else knows. If you want to keep your Boss, Buddies, FBI, Lovers, informed of something, but don't want any of the recipients to know, use BCC.
If one of your recipients is good at hacking, don't bother with BCC, just copy the info into a new, totally separate email, and send that.