Tethered is where you have the cable to plug into your car attached to the charger already. You just unwrap the cable from around the charger and plug it straight into your car. Untethered is when you have a separate cable that needs to be plugged into the car and the other end plugged into the charger. This is really a personal choice on which you prefer or if you have a preference on how the charger looks when it is not in use (some people don’t like a cable hanging or wrapped around the charger when it is not in use).