Collectible coins, particularly in the United States, often include various types such as proof sets, commemorative coins, and bullion coins. Proof sets, like those from 2018, are specially minted coins that are produced with a higher level of detail and a polished finish, making them ideal for collectors. The 25-cent coin (quarter) is a popular denomination, and proof versions of these coins often feature unique designs and limited mintages, enhancing their collectible value. Collectors seek these coins for their aesthetic appeal, historical significance, and investment potential.