ERC is an acronym for "Ethereum Request for Comments". ERC provides technical guidance to developers for construction of different tokens.
ERC-721 smart contracts are non fungible. This means that they are unique and are not interchangeable. They need a new smart contract for each type of token.
Doodles, one of the most popular NFT collections, is an ERC-721 smart contract. Each Doodle has its own traits that make them different from other Doodles.
ERC-1115 smart contracts are BOTH fungible and non fungible. They support an infinite number of tokens. ERC-1155 collections are usually games or trading cards where you can buy more than one of the same item.