In the context of collectible coins, the 2011 United States Mint Proof Set includes a selection of coins that are characterized by a mirror-like finish, created through a special minting process. The set typically contains the five circulating coins, including the quarter (25 cents), along with commemorative issues. Each coin in the set is encapsulated for protection and often features the "S" mint mark, indicating it was produced at the San Francisco Mint. Collectors value these sets for their quality and limited mintage, making them a desirable addition to numismatic collections.