ERC-721 is a free, open standard that describes how to build non-fungible or unique tokens on the Ethereum blockchain. While most tokens are fungible (every token is the same as every other token), ERC-721 tokens are all unique.
Think of them like rare, one-of-a-kind collectables.
A standard interface for non-fungible tokens, also known as deeds. The standard allows for the implementation of a standard API for NFTs within smart contracts, and provides basic functionality to track and transfer NFTs.