NFTs are part of the Ethereum blockchain so they are individual tokens with extra information stored in them. That extra information is the important part, which allows them to take the form of art, music, video (and so on), in the form of JPGS, MP3s, videos, GIFs and more. Because they hold value, they can be bought and sold just like other types of art – and, like with physical art, the value is largely set by the market and by demand.