On many devices, the IMEI number can be retrieved by entering *#06# or is located in the battery compartment. Android : Settings --> About Phone iOS : Settings --> General --> About Sony Ericsson (New Devices) : Options --> status. Sony Ericsson (Old Devices): Dial * Right * Left Left * Left Blackberry : [...]

An Unlock Code is a numeric string. When entered into a locked handset it releases the SIM lock restriction and allows the handset to be used on other GSM carriers.

Insert a SIM Card from a different carrier than the one your handset is using. When power up is complete the handset will display a message like those below if it SIM locked: Enter Subsidy Code, Enter Unlock Code, Incorrect Sim Card, Insert Correct Sim Card, Invalid Smart Chip, Phone Restricted, [...]

The Digital Millennium Copyright Act, introduced an exemption on November 27, 2006, specifically allowing unlocking of mobile devices.