Crypto glossary

ERC-721

ERC-721 is the Ethereum standard for non-fungible tokens (NFTs), where every token has its own unique ID and is not interchangeable with any other token in the same contract.

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How ERC-721 works

In an ERC-20 contract, the contract tracks how many identical units each address holds. In an ERC-721 contract, it tracks who owns each individual token ID. Token 1 and token 2 in the same collection are different objects, even if they look alike.

The standard defines functions such as ownerOf (who holds a given ID), transferFrom and safeTransferFrom (move a specific token), and approve or setApprovalForAll (let another address, such as a marketplace, transfer your tokens). It was finalized in 2018, after the CryptoKitties game in late 2017 showed how popular unique on-chain items could be.

Where the picture actually lives

Most ERC-721 tokens do not store the image on the blockchain. Instead, each token has a tokenURI, a link to a metadata file describing the item, which in turn points to the image or media. That file might be on decentralized storage such as IPFS or on an ordinary web server.

If the metadata sits on a server that later shuts down, the token still exists on-chain but may point to nothing. Some collections store everything on-chain to avoid this, at a much higher cost.

What ownership means

Holding an ERC-721 token proves you control that token ID in that contract. It does not by itself give you copyright to the artwork; any rights depend on the license the creator attaches. And the contract owner may have powers such as changing the metadata link.

Risks to watch

The setApprovalForAll function is a common phishing target. Signing it for a malicious site lets that site move every NFT you hold from that collection. Fake collections also copy names and images of real ones, so check the contract address on the project's official channels.

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Frequently asked questions

What is the difference between ERC-721 and ERC-20?

ERC-20 tokens are interchangeable units, like money. ERC-721 tokens each have a unique ID, like a numbered certificate.

Is ERC-1155 the same as ERC-721?

No. ERC-1155 is a later standard that lets one contract manage many token types, both fungible and non-fungible, and batch transfers efficiently.

Can an ERC-721 token be copied?

The image can be copied by anyone, but the token ID in the original contract cannot be duplicated. A copy would be a different token in a different contract.

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All glossary terms · Educational reference only — not investment, legal, tax or financial advice.