ERC-721 or Non-Fungible Token (NFT) is used to identify something 
or someone in a unique way. It refers to a token standard that can
be found on the Ethereum blockchain.

NFTs are tokens that have unique characteristics, setting them 
apart from other tokens.

References:

https://ethereum.org/en/developers/docs/standards/tokens/erc-721/
https://coinmarketcap.com/alexandria/glossary/erc-721