NFC Tags are small chips which you can get in the form of stickers, cards, rings or even keychain. They can store a small amount of data in the range of hundreds of bytes. These are passive things which can work without a power supply or rely on other devices for power such as an NFC Tag depends on your Phone or the NFC readers to work.
Here I have bought a set of 5 tags for 4$ (₹250) from Amazon. The NFC tags I have can store data up to 144 bytes. There are different types of NFC tags which can store a larger amount of data.