ERC721 tokens are Ethereum non-fungible tokens. Non-fungible means that the value of each individual token is usually different from others. For example, if you have 10 EMB (ERC20 tokens), the USD and ETH value of your portfolio value is known and depends on EMB/ETH rate.

If you have 10 Cryptokitties, the value of your portfolio depends on how much the users are willing to pay for each kitty: one creature can be worth 0,002 ETH and another would be 300 ETH. The price is defined by the seller themself, so ERC721 tokens cannot be traded on exchanges.