ERC-721 is a free, open standard that describes how to build non-fungible or unique tokens on the Ethereum blockchain. While most tokens are fungible (every token is the same as every other token; for example, one bitcoin is always equal to another bitcoin), each ERC-721 token is unique and can be owned by individuals and transacted in peer-to-peer transactions.
ERC-721 tokens are used to represent ownership over unique items, like a particular in-game item or a specific piece of real estate. They could also be used to prove ownership of digital content, such as digital art or even virtual pets. The ERC-721 standard provides a mapping of unique identifiers (each token) to addresses (owners).
One notable aspect of ERC-721 is that the standard includes an optional metadata extension, which allows for each token to have associated metadata (like the name of the item, a description, and an image). This can be useful in making the tokenized asset more understandable and accessible, particularly in graphical interfaces.
The ERC-721 standard was made famous by CryptoKitties, a game on Ethereum where each “CryptoKitty” is an ERC-721 token and is therefore unique.
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