ERC, which stands for Ethereum Request for Comment, is a standard conveying the essential technical notes and requirements to a group of developers and users. ERC-721, initially proposed by Dieter Shirley as an Ethereum Improvement Proposal (EIP), is the standard for representing ownership of non-fungible tokens (NFTs) on Ethereum.
The ERC-721 token standard allows for the creation of NFTs that can be associated with a physical property, like land and artwork. When an ERC-721 token is created, each token is cryptographically unique, so no other token with the same characteristics exists alongside it.
CryptoKitties was the first project to adopt the ERC-721 standard for its collection.