(For reference during this section, the standard can be found at the following link: https://github.com/ethereum/eips/issues/721)
The tokens we've discussed so far have all been based on the idea of value, where that value was initially defined at the time of crowdsale in terms of an existing currency: you buy a given number of tokens for a given amount of ETH.
It's possible, however, to allow tokens to represent other things, such as physical objects or a reputation. In contrast to an abstract unit of value, such as a dollar, bitcoin, or ether, such things aren't fungible. Fungibility is the characteristic of an asset that determines whether assets of the same type can be considered interchangeable during an exchange. One ether has the same value and utility of any other ether, and so they are considered completely interchangeable and fungible. One item, such as a car, a baseball card, or a CryptoKitty (https://www.cryptokitties.co/), is not directly interchangeable with another, so is considered non-fungible. ERC-721 was proposed to tokenize such non-fungible assets.