A paper wallet is an offline document that contains all data needed to generate multiple private keys. Paper wallets can be used to store bitcoins (or any other type of digital currency). Software-based paper wallets are called "client-side", because the user must download and install them on their own computer before using them (the program with its HTML files serves as an interface for managing your crypto coins). Alternatively, hardware-wallets (such as TREZOR) can also generate paper wallets.