ERC-721 is the standard for tokens on the Ethereum blockchain, which are unique and do not have interchangeability properties.
Let us consider an example. The main number of tokens in the Ethereum network are issued on the ERC-20 standard. They have the property of interchangeability and can be compared with money. That is, a dollar bill can always be replaced with exactly the same. So with the ERC-20 tokens, such digital coins have the same characteristics and value.
ERC-721 tokens are distinguished by the fact that each coin has its own unique characteristics and therefore it is correct to compare them not with money, but with collectible items. Everyone learned about this standard of tokens after launching the acclaimed blockchain game CryptoKitties. Due to the use of the ERC-721, each cryptoken is unique and has its own price.
The lack of interchangeability of ERC-721 tokens creates special conditions for their pricing. That is, for a fan of CryptoKitties, the cost of a particular cryptoto can be tens of thousands of dollars. And for those who are not interested in this game, the price of such a token is zero.