The IMEI (International Mobile Equipment Identity) is a unique number to identify GSM, WCDMA, and iDEN mobile phones, as well as some satellite phones. Mostly phone have one IMEI number, but in dual SIM phones are two.
The IMEI is only used for identifying the device and has no permanent or semi-permanent relation to the subscriber. 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.
To view your IMEI dial *#06# or look in the settings of your device.