Paper with a special dry ink or pigment coating, carbon paper is typically sandwiched between two or more sheets of plain paper or pre-printed forms to form a carbon set for making duplicate copies of an original handwritten or typed document. When writing or typing on the top sheet or form the pressure of the pen or typewriter causes the carbon on the carbon paper to rub off and leave an impression or carbon copy on subsequent pages in the carbon set or other surfaces.
Carbon paper and carbon sets have been mostly replaced by either carbonless copy paper or photocopying. The carbonless paper, also called NCR paper after the company that created it, works in much the same way as carbon paper except the papers in the set are coated with special dyes and clays that transfer the image without the interleaved carbon paper. With computers replacing typewriters, these carbonless sets are mostly used when on-the-spot duplicates of handwritten material is needed (such as receipts, parking tickets, or duplicate checks). The development of copy machines makes it easy to photocopy original handwritten, typed, or computer printed letters and forms without the need for carbon paper.
The term cc often used on business letters originally stood for carbon copy and it was an indication that someone had received a carbon copy of the original. Today it is generally understood to mean courtesy copy although the phrase carbon copy still persists.
Transfer Paper for crafts is another use for carbon paper. (Not to be confused with iron-on transfer paper.) It's used to transfer patterns onto fabric, contact paper, and other surfaces.