In mobile devices, the PIN acts like a password preventing other people from gaining unauthorized access to your device. This is a numeric code which must be entered each time the device is started (unless the PIN security feature is turned off).
In GSM mobile phones, the PIN is normally associated with the SIM card (not the phone) and must be entered each time the phone is switched on. If a wrong PIN is entered three times in a row the handset is locked until you enter another code, called a PUK code.
Both the PIN and the PUK codes are supplied by the operator, but only the PIN code can be changed by the user.