ERC-721

ERC-721 tokens contrast to ERC-20 tokens as in ERC-721 tokens are non-fungible. This means that every token is unique and subsequently, not interchangeable.

Breaking down ERC

ERC represents Ethereum Request for Comments. This isn't a technology or platform, it gives technical guidance to engineers to construction. As of December 2018, there are nine last Ethereum Requests for Comments. ERC-20, ERC-55, ERC-137, ERC-162, ERC-165, ERC-181, ERC-190, ERC-721, and ERC-1167.
Engineers can make an ERC by presenting an Ethereum Improvement Proposal (EIP).
ERC-721 - The Non-Fungible Token Standard, was made by William Entriken, Dieter Shirley, Jacob Evans, and Nastassia Sachs due to ERC-20 interface ineptitudes. Through an Ethereum Improvement Proposal (EIP) made on the 24th of January 2018.
A famous Ethereum decentralized application, Cryptokitties, used ERC-721 to make unique digital collectibles as kitties.
Each unique cat was worth an alternate amount on the marketplace, directed by the users of the platform. Each unique little cat orders an alternate price, and subsequently, every token should be unique.
The creation of blockchain based non-fungible tokens takes into consideration

• physical property (houses, work of art, and vehicles)
• Virtual Collectibles (Cryptokitties, rare pepes, collectible cards)
• Assets with negative value (loans)

To make an ERC-721 token, the contract must be consistent with both the ERC-721 and ERC-165 interfaces.