Collectible coins include various types such as Proof Sets, which feature specially made coins with a high-quality finish, intended for collectors rather than general circulation. In the context of the United States, the 2020 Proof Set includes coins with a 25-cent denomination (25c), showcasing refined details and mirror-like surfaces. These sets commemorate specific themes or events and are often produced in limited quantities, enhancing their collectible appeal and potential value in numismatic markets.