JailBreak

Jailbreak

Jailbreaking iPhones and iPads are common among Apple users who want to install apps that aren't available through Apple's official channels.

Apps are frequently posted on Cydia and other jailbroken device download sites by developers who don't want to go through Apple's review process or follow Apple's AppStore guidelines.

Jailbreaking can also be used to get around Digital Rights Management (DRM) and share copyrighted media and gain access to restricted file systems, user interfaces, and network capabilities.

Tethered jailbreaks don't persist through reboots, so re-applying jailbreaks requires the device to be connected (tethered) to a computer during every reboot. The device may not reboot at all if the computer is not connected.
Semi-tethered jailbreaks can't be re-applied unless the device is connected to a computer during reboot. The device can also boot into non-jailbroken mode on its own.
Semi-untethered jailbreaks allow the device to boot on its own, but the kernel patches (or user-land modifications) for disabling code signing aren't applied automatically. The user must re-jailbreak the device by starting an app or visiting a website (not requiring a connection to a computer, hence the term untethered).
Untethered jailbreaks are the most popular choice for end users because they need to be applied only once, after which the device will be permanently jailbroken.