ERC-721 is a standard for building non-fungible tokens on the Ethereum blockchain. ERC-721 tokens are non-fungible tokens. It defines a minimum interface a smart contract must implement to allow non-fungible tokens to be managed, owned, and traded on the Ethereum network.
While ERC-20 is a class of identical tokens, ERC-721 is a class of unique tokens. All ERC-721 tokens are unique.
CryptoKitties uses ERC-721.
The ERC-721 standard is free and open source.