http://erc721.org/ (Edit: https://cointelegraph.com/explained/non-fungible-tokens-expl... is probably better) has a nice friendly writeup. It's basically a token on Ethereum representing a unique object such as artwork, media (like above), a cool sword in a game, cards in a trading card game (Magic like games such as God's Unchained), or even physical items such as cars, houses etc. These tokens represent an individual thing that needs to be usable in the digital world (sending to others, selling/trading, using for identity management etc).