Collectible coins encompass a variety of types, including numismatic coins, bullion coins, and historical coins. Specifically, the 5-cent nickel produced in 1938 at the Denver Mint represents a significant year, as it marks the first year of the Jefferson nickel series, which replaced the Buffalo nickel. The Denver mintmark (D) indicates its production location, and these coins are sought after by collectors for their historical significance and condition. The 1938-D nickel can vary in value based on its grade and rarity in the market.