CC originally meant Carbon Copy. Before there were photocopiers and computers, if you wanted to make a copy of a document, you would take a piece of carbon paper and stick it between two pieces of typing paper (plain white paper). The carbon paper would have a powdery black substance (the "carbon" stuck to one side which you would face towards the bottom piece of typing paper. You take this paper "sandwich" and roll it into your typewriter. When you typed on the top sheet, the impact of the metal type would cause the black carbon to be transferred from the carbon paper to the sheet underneath. If you needed more than one copy, you just stacked another piece of carbon paper and typing paper underneath. If you made a mistake, you had to roll everything out, take apart your paper sandwich, erase the mistake from each copy (or, in later years, white it out), put the sandwich back together, roll everything back into the typewriter, and continue. Carbon Copy doesn't really have the same connotation as it did back then, so some people started calling it Courtesy Copy, even though it has the same purpose.
The original should go to the primary person or people to whom it is addressed. Carbon copies are routed to people who need to be kept in the loop as far as what is going on. For example, if you are writing a letter to a client, you may want to CC your boss so your boss knows what is going on.
And there's also something called a BCC, or Blind Carbon Copy. This is something that is unique to e-mail. Everyone on the BCC line will get a copy of the e-mail, but their e-mail address will not appear on everyone else's copy, so no one else will know that they got the e-mail. This is useful if you want to protect the privacy of the recipient.