Smartphones present a slightly different unlocking scenario. They're essentially handheld mobile computers. Locking and unlocking them is a more involved process than the simple linking of serial numbers and account numbers seen in regular cell phones.
Smartphones are often locked to a service provider. For example, the iPhone is locked to the AT&T network. For some smartphone owners, unlocking isn't a matter of wanting to change service providers. These owners just want to gain control over the applications they can install on their device. A lot of smartphone manufacturers lock the devices so that only approved applications can be installed. The iPhone can only install apps purchased from Apple's App Store, for instance. If there's an application you want to install that isn't officially approved and offered by Apple, you're out of luck. That is, unless you can unlock your smartphone.
It's possible to purchase unlocked iPhones, or you can apply a software crack to an iPhone to unlock it. This is often referred to as jailbreaking the phone. Because the software on a smartphone is more complicated than a cell phone's, the unlocking process is more difficult than simply entering a code. The software cracks can have unpredictable results, rendering some features of the phone (or the phone itself) non-functional.
Is all this unlocking and jailbreaking legal? In the next section, we'll find out.