IMEI = International Mobile Equipment Identity.A 15-digit number which includes information on the origin, model, and serial number of the device. The model and origin comprise the initial 8-digit portion of the IMEI, known as the "Type Allocation Code/TAC". The remainder of the IMEI is manufacturer-defined, with a "Luhn check digit" at the end (which is never transmitted). The "Luhn check digit" is calculated from the rest of the IMEI.
It should be noted that in SE-based phones, the IMEI is stored in two places, the OTP (One Time Programmable memory) and GDFS. The GDFS IMEI is normally read from the OTP, but this can be circumvented by SETool function to "change" the IMEI. This patches the firmware into allowing different OTP/GDFS IMEIs. It is the GDFS IMEI that is reported to the network, so changing this will "de-bar" blocked phones. SEUS is not fooled by this, on the other hand, and it should also be noted that doing this is illegal in most countries.