The International Mobile Equipment Identity or IMEI is a number, usually unique, to identify GSM, WCDMA, and iDEN mobile phones, as well as some satellite phones. It is usually found printed inside the battery compartment of the phone. It can also be displayed on the screen of the phone by entering *#06# into the keypad on most phones.
The IMEI number is used by the GSM network to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network in that country. For example, if a mobile phone is stolen, the owner can call his or her network provider and instruct them to "blacklist" the phone using its IMEI number. This renders the phone useless on that network and sometimes other networks too, whether or not the phone's SIM is changed.