A SIM card is a tiny chip found inside most cell phones, but most of you are probably aware of this. We're also guessing that you know how it can be used to store contact information and a small number of text messages.
Yet did you know that a tiny amount of memory on the SIM card allows for program code to be stored there? Basic applications are usually pre-written on this program space by the carrier issuing the SIM Card. This allows users to perform operations related to their wireless service – to check their balance, to enable service features, stuff like that.