NFT stands for non-fungible token, a type of smart property. They initially saw mainstream success with CryptoKitties and have since taken over popular culture; some of the world's most famous artists have begun tokenizing their work, and many celebrities are now producing their own. The most popular NFT platforms are Ethereum (usually ERC-721 or ERC-1155 with decentralized marketplaces like OpenSea), Flow, and WAX, and most blockchain games use them.
Fungibility refers to whether or not all assets in a set are interchangeable. For example, while one US dollar is equal in value to another, things like cars or homes vary in value according to age and model. Fungible assets are generally more liquid (easier to sell) than non-fungible assets, since buyers can be less picky about which specific token they are getting. Non-fungible tokens have more information about them, including off-chain files (such as image files for art) and on-chain metadata, which differentiates them from one another.
NFTs are the most widely recognizable implementation of the true ownership concept in gaming; blockchain-based collectible pets, items, weapons and armor, or land are considered NFTs because they have varying names, artwork and attributes. Blockchain games commonly presell them before launch in order to raise funds for development, instead of or in addition to an ICO. Countless artwork NFTs have also been sold, one for as much as $69 million.