ERC-721 is a standard for creating non-fungible tokens (NFTs) on the Ethereum blockchain. Unlike cryptocurrencies like Bitcoin or Ethereum itself, which are fungible and can be exchanged one-for-one, ERC-721 tokens are unique and represent ownership of specific digital assets, such as art, collectibles, or virtual real estate. This standard allows developers to create distinct tokens that can have different values and properties. Each ERC-721 token has a unique identifier, making it possible to verify ownership and provenance. This has led to a booming market for digital collectibles and has transformed how we think about ownership in the digital world.