Unlocked iPhones can be used with different GSM carriers—the choice is yours. A new device is factory-unlocked, which can result in a higher price, compared to the locked iPhones. The latter are subsidized by mobile carrier companies.
Note: Jailbreaking a factory-unlocked iPhone is essential for those who want to maximize their benefits. As you may already know, jailbreaking may cause certain damages, if it is not done correctly.
Nevertheless, if you are looking for a device that would give you the freedom of switching carriers or using the foreign ones, an unlocked iPhone is exactly what you need.