What is Factory Unlocked iPhone

FU (Factory Unlocked) iPhone is an iPhone which officially unlocked by Apple. This model is permanently unlocked. Neither software hack nor turbo-sim needed after restore/update iPhone firmware. You can use it as a phone anywhere there is a carrier that supports GSM or UMTS/HSDPA 3G format with any SIM-card.
You don’t need to Unlock your iPhone whenever its software get updated.
The IMEI number has been registered and marked as Unlocked in Apple’s database. Hence, you can enjoy to update firmware without need to worry if your iPhone will be re-locked again.

Visit this iPhone support to see all carriers which is authorized to sell factory unlocked apple iPhone.