IMEI
IMEI stands for International Mobile Equipment Identity. The 15-17 digit IMEI number is unique to each mobile device and identifies that device. The IMEI is used to allow or disallow access. For example, your cellular provider can blacklist a stolen device based on the IMEI number. When you purchase a mobile device you can also use the IMEI to ensure it’s not locked and is available to be activated on your carrier’s network.
IMSI
IMSI stands for International Mobile Subscriber Identity. The IMSI identifies the user of the mobile network. The 15 digit (or less) IMSI number is usually programmed into the SIM card. This means you can move your SIM card between different phones and still maintain access to your wireless provider’s network.