Similar to ERC-20, ERC-721 (Ethereum Request for Comments 721) is another Ethereum standard used for representing the ownership of Non-Fungible Tokens (NFTs) with other unique tokens that are verifiable on the Ethereum blockchain system.

While the ERC-20 standard is used for fungible tokens, ERC-721 smart contracts are used to map the unique token ID of an owner rather than just checking balances. When a wallet contains multiple NFTs, ERC- 721 ensures that they are individually unique and their token IDs point to different owners.

The developers must follow these standards during the creation of NFTs so that they can be compatible with various applications such as exchanges, wallets, and decentralized applications.