It's short for Cardano NFT and helps differentiate from other NFTs in the blockchain space. 

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What is an NFT?

Non-fungible token.

“Non-fungible” more or less means that it’s unique and can’t be replaced with something else.

NFTs can be used to represent items such as photos, videos, audio, and other types of digital files.

Stored on a digital ledger, called a blockchain, that certifies a digital asset to be unique and therefore not interchangeable.

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Where do I store my NFTs?
In your Cardano wallet.