ERC-721

ERC-721 is the token standard on Ethereum for unique tokens, items that are each one-of-a-kind rather than interchangeable. If ERC-20 tokens are like identical dollar bills, ERC-721 tokens are like numbered tickets, deeds, or trading cards: each one is distinct and not simply swappable for another. These are the tokens behind non-fungible tokens (NFTs), used to represent a specific digital artwork, a collectible, an in-game item, or a claim to a particular asset. The standard exists so that all these unique items can be owned, shown, and traded through the same shared tools.

Where an ERC-20 contract just tracks how many tokens each address holds, an ERC-721 contract gives every token its own identifying number and records exactly which address owns that specific one. The standard defines functions to look up the owner of a given token, to transfer that particular token to someone else, and to authorize another party to move it for you. Each token usually points to extra details, such as an image and description, stored elsewhere, so a marketplace can display the right picture. Ownership of that exact item, however, is settled on the chain itself.

ERC-721 matters because it gave digital uniqueness a common, portable form. Because so many platforms adopted it, an NFT created in one app can be viewed, displayed, and traded in countless others, and ownership transfers in the open rather than inside one company's private database. It turned the idea of a verifiable, transferable one-of-a-kind digital item into a shared standard that the whole ecosystem can build on.