Unlocking an iPhone is often interchanged with jailbreaking. But they're not exactly the same. Jailbreaking is a procedure that's done to let you use applications that are not Apple-approved on your iPhone. However, before you can unlock your iPhone, you need to jailbreak it. Unlocking in this sense means smashing the lock on your SIM card and enables you to use your iPhone with not just carriers which are approved by Apple, however with whichever network you prefer.