Short version:

NFTs are digital goods or assets.

Here are just some examples of digital assets that could nowadays be considered NFTs:

• Hats, tools inside Team Fortress 2
• Weapons skins inside Counter-Strike: Global Offensive
• League of Legends and DotA 2 skins

NFT categorization by ERC standards:

ERC20 - Technically speaking, ERC20 does not qualify as Non-Fungible Token. However, ERC20 is a well known standard used for creating cryptocurrency smart contracts and thus can be used for in-game currencies. Also, it can be used for Rewards and Achievements. ERC721 - Unique items, collectibles or land ownership. ERC1155 - Consumables.