What Is ERC-721?
Also known as: EIP-721, NFT Standard, Non-Fungible Token Standard
In short
ERC-721 is the standard that defines how tokens that each have a unique ID number and are not interchangeable, that is, NFTs, are created and transferred on Ethereum; it is the core standard for digital art and collectible NFTs.
In ERC-20, all token units are identical to one another; in ERC-721, each token is distinguished by a tokenId and has its own owner. The standard defines functions such as ownerOf (querying the owner), safeTransferFrom (safe transfer), approve and setApprovalForAll (granting permission for the entire collection). The tokenURI function in the optional metadata extension returns the address of the file containing the NFT's name, description and image.
The standard was formalized after the CryptoKitties game, which drew great interest in 2017, and took its final form in 2018. Digital artworks, profile picture collections, ENS domain names, in-game characters and event tickets are examples of ERC-721 use. ERC-1155, which manages multiple token types in a single contract, is an alternative developed later.
The NFT's image and metadata are generally stored not on the blockchain but in distributed storage systems such as IPFS or on centralized servers; when a centralized server shuts down, the content can become inaccessible. In addition, phishing sites obtaining a setApprovalForAll approval from a user to steal the entire collection is a common scam method.
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