Why Jailbreak or “Root” Your Mobile Device?
Jailbreaking is the act of overriding the software limitations on the iPhone, iPod Touch, or iPad. Similarly, on an Android device, the terms “rooting” means giving yourself root permissions on your phone. It’s similar to running programs as an administrator in Windows, or running a command with
sudo in Linux. With a rooted phone, you can run more apps (like backup or tethering apps), as well as flash custom ROMs to your phone, which add all sorts of extra features. For privacy, you might want to run a continuous VPN or SSH tunnel.
Is It Legal To Do This?
Yes, it is. On July 26 2010, the U.S. Copyright office announced a new exemption making it officially legal to root a device and run unauthorized third-party applications, as well as the ability to unlock any cell phone for use on multiple carriers.
Lifehacker has a comprehensive guide to jailbreaking your iPhone.
Rooting Android Phones
Lifehacker is also place to find the comprehensive guide to rooting your Android device.
You can also run Debian in a chroot environment on Android, but this still requires running the full Android stack. Here is a step by step guide.