untethered jailbreak is new jailbreaking technology. In this jailbreaking system you don't want to reboot your iphone and connect it to an external device. Redsnow unterthered jailbreak means use the redsnow for this kind of jailbreak
You can use Redsnow unterthered jailbreaking system for all apple idevices just like iPhone, iPod or iPad. Many firmware supports for the unterthered jailbreak. We recommend to this jail breaking if you only have iOS 4.2. Otherwise select the normal jailbreaking system to your device.