ERC-721
The Ethereum standard for non-fungible tokens, where every token has a unique identifier and exactly one owner.
ERC-721 defines how to ask who owns a specific token identifier, how to transfer it safely to a contract that knows how to receive it, and how to find the token's descriptive metadata through a link the contract returns. It was finalized in 2018 and became the template for collectible and per-item token design. The metadata link normally points to a web address or a content-addressed storage network, so the durability of what a token displays depends on where that file lives. The related ERC-1155 standard lets one contract manage many identifiers with quantities, which suits items that are neither purely unique nor purely interchangeable.
In practice
A block explorer can show the current owner of a single collectible because ERC-721 requires the contract to answer an ownership query for each token identifier.
The common misunderstanding
An ERC-721 contract usually stores only an identifier, an owner, and a link; the image itself is almost never held on the blockchain.