ERC-721 tokens are non-fungible tokens (NFT) based on Ethereum.
ERC-721 - The Non-Fungible Token Standard, was developed in 2018 by William Entriken, Dieter Shirley, Jacob Evans, and Nastassia Sachs, and introduced on January 24, 2018 through an Ethereum Improvement Proposal (EIP) to address ERC-20 interface issues (ERC-20, also known as smart contracts, are exchangeable, "fungible," tokens. However, the goal of the ERC-721 token standard was to tokenize unique digital collectibles: NFTs)
ERC stands for "Ethereum Request for Comments." It is neither a technology nor a platform, but a technical blueprint for developers. In addition to the ERC-721 standard, there are currently eight other common ERCs: ERC-20, ERC-55, ERC-137, ERC-162, ERC-165, ERC-181, ERC-190, and ERC-1167.
Developers can either build on these standards when designing new coins and tokens, or submit an Ethereum Improvement Proposal (EIP) to establish a new ERC for a use case not previously covered.