Walter's answer is good, however, Peachy's answer is more technically correct. The "cc" letters do stand for "carbon copy" an old process of placing a piece of carbon paper between two pieces of blank paper and drawing, hand-writing or typing, thereby creating a copy on the bottom piece of paper. The pressure on the top piece of paper would cause the ink (carbon) on the carbon copy paper to transfer ink onto the second (bottom) piece of paper. This was a common way of making copies up until about 1960 or 1970. Anyone under 30 may not have heard of this technique. I have read that it was invented by someone in England around 1800.