ERC-721 pertains to a token standard meant for Non-Fungible Ethereum tokens. This token standard had been introduced through an Ethereum Improvement Proposal back in 2017. This enables smart contracts to operate as tokens that can be traded as with ERC-20 tokens.
The ERC-721 standard allows for the implementation of a standard API for NFTs within the context of smart contracts. What this provides is the basic functionality to track and transfer Non-Fungible Tokens. The creation of this standard had largely considered the use cases of NFTs owned and transacted by individuals alongside the consignment to brokers, wallets, and auctioneers who serve as third-party entities.
A standard interface permits wallets, broker, and auction applications to function with any NFT on Ethereum blockchain. The standard was inspired by ERC-20 and furthers its industry experience since the EIP 20 was launched.