iOS JailBreaking

       Jailbreaking is a process that allows iPad, iPhone and iPod Touch users to gain root access and unlock the operating system thus removing any limitations imposed upon them by Apple. 
      Once jailbroken, iPhone users are able to download many extensions and themes previously unavailable through the App Store via installers such as Cydia. A jailbroken iPad, iPhone or iPod Touch is still able to use the App Store and iTunes.

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 origin. 

     Jailbreaking, according to Apple, can void Apple's warranty on the device, although this is probably  an unlawful restriction under the United States Magnuson–Moss Warranty Act. It can be quickly remedied by restoring the device in iTunes.

     Cydia founder Jay Freeman estimates that over 10% of all iPhones are jailbroken.