ERC-721

ERC-721 is the standard most people mean when they casually say "NFT." It defines how unique token IDs are created, owned, transferred, and described so wallets and marketplaces can work with many collections in a consistent way.

The standard does not force every project to be identical. It gives common hooks and expectations. Once those are present, outside applications can ask who owns a token, move it with approval, and fetch the metadata that explains what it is.

In practice, wallets and marketplaces rely heavily on these standardized methods. If a project breaks expectations around ownership or metadata resolution, users may see confusing behavior even when the contract technically compiles and deploys.

For learners, ERC-721 is a good first mental model because it ties together three ideas: uniqueness, transferability, and metadata lookup. Most NFT mechanics are variations on that foundation.