What information can you store ?
There's a whole set of different data types you can store on an NFC tag. The actual amount of data varies depending on the type of NFC tag used - different tags have different memory capacities. For example, you may choose to store a URL (web address) or a telephone number. A standard Ultralight nfc tag can store a URL of around 41 characters, whereas the newer NTAG203 nfc tag can store a URL of around 132 characters.
Usually, this information is stored in a specific data format (NDEF - NFC data exchange format) so that it can be reliably read by most devices and mobile phones.