By treating the die in a special way, the coins it produces have a different appearance.  Modern technology allows the high points on the coin design to be acid treated (on the die).  The background (field) design of the coin die is polished, resulting in a mirror-like look on the coin it strikes. This gives the finished coin a frosted look (frosting) on the raise parts of the design, with a mirror like finish on the background. This contrasting finish is often called "cameo".  On some older coins a cameo appearance is quite rare. The attribute "CAM", when added to a coin's description, means cameo appearance. "DCAM" means deep cameo, and indicates the cameo appearance is strong and easy to observe.

Proof coins are struck twice, or more!
 

Not only are proofs made using specially treated dies, each coin is struck two or more times by the coin die. By striking it more than once the metal is forced into all the crevices of the die, thereby giving a very fine detail to the image on the coin.  This fine detail does not appear on most non-proof coins.  Proof Coins prove to be more valuable in most cases.