ERC-721 is a technical standard for token implementation using smart contracts on the Ethereum blockchain. ERC-721 standards function similarly to ERC-20 standards in that they provide a common set of rules for the purpose of simplifying the creation and ensuring the functionality of tokens. This also allows most wallets supporting Ether and ERC-20 tokens to support ERC-721 tokens as well. However, ERC-721 tokens differ from ERC-20 tokens in that they are non-fungible; that is, the tokens cannot be divided or interchanged with all tokens of the same type. Non-fungible tokens are analogous to baseball cards, as fungible tokens are to dollar bills.
The framework for ERC-721 tokens was first published as part of an Ethereum Improvement Proposal in September 2017. The most notable use case of ERC-721 tokens (and NFTs in general) is the CryptoKitties collectable game, in which users purchase and ‘breed’ unique virtual cats of varying degrees of rarity. At one point, CryptoKitties accounted for 12% of all transaction volume on Ethereum.