"NFT" is a Crypto Currency term for a one-of-a-kind collectible art object, that is stored as an image file on one of the "Blockchains".
A Blockchain is a cloud based ledger database of all things related to CryptoCurrency. There are several Blockchains, each usually developed by the developers of a specific Cryptocurrency "Coin".

My favorite Blockchain is the Polygon Blockchain, and Polygon has a popular Coin named Matic, very closely related to the more widely known coin "Ethereum". NFTs are most commonly associated with the Ethereum coin, and have been throughout the past 4-5 years of NFT popularity, because the Erc721 (Ethereum Request for Comment protocol or standard #721) laid out the technical scope of the javascript code design of a NFT listed on a Blockchain.

There is only one "copy" of a Erc721 NFT. A .gif, .jpeg, .png, .mp4, etc image file.
There is only one Erc1155 NFT file, but there can be an unlimited supply of copies, referred to as "serial numbers". Like in real world art, when the artist prints 100 copies of their work, but signs them as "Nude On A Stool" 1/100, etc.

I have a Marketplace, which helps me Create, Mint, List and Sell my erc721 and erc1155 NFTs.
If a buyer comes along wanting to buy one of my NFTs, they first need to register with the marketplace, by signing in with their Crypto Wallet, oftentimes MetaMask, but there are others. They then can click on the appropriate buttons and links to buy the NFT, and the Blockchain is updated to move ownership of that file (reference) from me to them. Not the file itself, but the ledger entry to it. Ultimately, the actual file is permanently stored on the IPFS - InterPlanetary File System! Somewhere in the "Cloud".

When I create and mint an NFT, I can state whether I want to collect royalties on future sales. If someone buys my 721 or 1155 NFT, I can earn 8% when they resell it to someone else, and 8% when that someone else resells to another someone else!