Collectible coins, particularly from the United States, encompass a wide range of types, including commemorative coins, bullion coins, and circulating coinage. The 25-cent piece, or quarter, has been produced in various designs, with special editions often included in Proof Sets. The 2001 Proof Set features coins struck with a higher quality finish, showcasing sharper details and a mirror-like surface, making them popular among collectors for their aesthetic appeal and limited mintage. These sets often include the state quarters introduced in that year, enhancing their desirability among numismatists.