Crypto glossary
ERC-721
ERC-721 is the Ethereum token standard for NFTs, in which each token has a unique ID and cannot be swapped one-for-one with another token of the contract.
ERC-721 defines a common set of functions for non-fungible tokens on Ethereum and compatible chains. Each token in a contract has its own ID, and the contract records which address owns each one. Standard functions let wallets and marketplaces check ownership, transfer tokens and grant another address, such as a marketplace, permission to move them.
The standard was finalised in 2018, after the success of CryptoKitties, and became the basis of most NFT collections, including Bored Ape Yacht Club and ENS names. A token usually points to metadata, such as an image and its attributes, stored on IPFS, on a company server or directly on-chain.
Owning an ERC-721 token proves control of a token ID, not copyright over the artwork, and if the metadata sits on a central server the image can disappear. Blanket approvals given to marketplaces are a frequent target of phishing.
Example
When an NFT with token ID 4521 is sold, the contract simply changes the recorded owner of ID 4521 from the seller's address to the buyer's.
