Jailbreaking is a process that allows iPad, iPhone and iPod Touch users to gain root access to the command line of the iOS operating system, thereby removing usage and access limitations imposed by Apple. Once jailbroken, iPhone users are able to download extensions and themes that are unavailable through the App Store (via installers such as Cydia) and perform other tasks that are not possible on store-bought devices, including installing non-Apple operating systems such as Linux. A jailbroken iPad, iPhone or iPod Touch running iOS can still use the App Store and iTunes and other normal functions, such as making telephone calls.
Jailbreaking is different from SIM unlocking, which, once completed, means that the mobile phone will accept any SIM without restriction on, for example, the country or network operator of the owners choice. Jailbreaking, according to Apple, can void Apple's warranty on the device. Under the DMCA of 2010, jailbreaking is perfectly legal in the United States, although Apple may claim it to void your warranty. It is also legal in many other countries including the EU. The jailbreaking process can be quickly and easily reversed by restoring the device in iTunes.