Tokens that can’t be exchanged for other tokens are considered digital assets. A non-fungible asset is one that cannot be exchanged for another of the same sort. An example of a fungible asset is Ethereum, which may be exchanged 1:1 with another Ether.
But an NFT is just a special kind of digital asset, and you can find the vast majority of them on the Ethereum blockchain. Additional NFTs have been implemented on several blockchains.