ERC-721

ERC-721 is the Ethereum standard for non-fungible tokens — one token ID, one owner, tracked individually. The interface is similar in spirit to ERC-20 (ownerOf(tokenId), transferFrom, approve) but built around unique IDs instead of balances. Each token carries a tokenURI pointing to off-chain metadata (usually JSON on IPFS describing name, image, traits) — the image itself is almost never stored on-chain because of gas cost. It's the standard behind every classic NFT collection: CryptoPunks (actually predates ERC-721 and inspired it), Bored Apes, Azuki, Art Blocks, ENS names. The limitation that pushed the ecosystem toward ERC-1155: ERC-721 is one contract per collection, and if you want a mix of unique and bulk items (a game with both rare swords and common potions), you need two standards working side by side.