ERC-721 (Ethereum Request for Comments 721) is a mandatory set of rules, functions, and events that a smart contract must implement to qualify as an NFT. Unlike the ERC-20 standard where all tokens are identical (fungible), ERC-721 requires each token to have a unique tokenId and a unique set of associated metadata, establishing its non-fungible nature.

Non-Fungible: Each token has a unique ID and is distinct from all others.

Key Function: The ownerOf(tokenId) function ensures there is only one verifiable owner for a specific token at any time.

Metadata: Links the on-chain token to external data (like an image, name, or property attributes) via a tokenURI.

Core Functions and Utility

The standard's functions ensure trustless and secure interaction with unique digital assets.

balanceOf(owner): Returns the number of unique tokens held by a specific address.

transferFrom / safeTransferFrom: Defines the secure methods for transferring ownership of a single unique token.

Approval Mechanics: Allows a user to delegate temporary control of their token to another address (e.g., an auction house smart contract).