In the most specific sense, a paper wallet is a document containing all of the data necessary to generate any number of Bitcoin private keys, forming a wallet of keys. However, people often use the term to mean any way of storing bitcoins offline as a physical document. This second definition also includes paper keys and redeemable codes. A paper key is a single key written on paper that is used multiple times like a wallet (this is strongly discouraged). A redeemable code is a single key intended to be funded and "redeemed" only once: these are commonly used for gifts and as part of physical Bitcoin coins/notes.But storing bitcoins on paper wallets is not safe unless very strict security precautions are undertaken during their initial preparation