The letters "cc" at the bottom of a letter or other correspondence stand for "carbon copy," or "courtesy copy" in modern terms. They mean that a copy of the correspondence has been sent to parties usually listed after the cc.
The use of the letters "cc" dates back to when the person writing the correspondence put a piece of carbon paper under the original to make a copy as he was writing. The "cc" lets the intended recipient of the letter know that a copy went to others as well. Because carbon paper is normally not used now, the meaning of the letters "cc" has changed to mean "courtesy copy," and they appear on emails as well as letters.