In most email clients you can send an e-mail to multiple recipients in the To: field by separating addresses by a semicolon.
You can also use CC to send a copy to other people. Is there any difference between the two? Do mail programs behave differently depending on whether you CC or multi-To?
It is described in RFC 2822, and more precisely section 3.6.3 of that RFC.
Here's the actual fragment that describes usage of "To" and "Cc":
The difference between "To" and "Cc" is in semantics, and determines how recipients should treat the message (as directed to them or "copied" only for their information).