What is tethered / untethered / semi-tethered?
A tethered jailbreak (or unlock) means that you have to connect your device to your computer again every time to boot it. You have to boot your device after it turned off. This can happen after the battery is completely empty of if you turn it off manually with the red slider. A tethered jailbreak is something you don't want. This is very nasty if you have to boot your device for some reason while you're on a trip. To boot your device (called tethered boot) you need a tool like redsn0w. Otherwise you just see the USB logo and you cannot use your device in any way.
Untethered is the contrary. You don't need a computer to boot your device. This is what you want.
Semi-tethered means you can still reboot your device without a computer, but the jailbreak is not active and some things won't work as expected (Safari and Cydia won't work). To get everything working again, you have to boot tethered again.
last edit: 2012-04-13
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