Unlocking vs. Jailbreaking
It is important that you understand the difference between the terms jailbreaking and unlocking. As you know, jailbreaking refers to our bypassing Apple's built-in operating system protections in order to grant full control over the device's hardware and software.
By contrast, unlocking means unlinking your iOS device from its current wireless carrier network. Thus, an unlocked Global System for Mobile Communications (GSM) AT&T iPhone 4S is then free to be used with another GSM carrier such as T-Mobile or Rogers.
The key distinction at play here is that the iPhone or iPad must first be jailbroken in order to be unlocked. Yes, we realize that the terminology here can get confusingbear with us.
In this series of articles, we discuss only the mechanics of jailbreaking, not carrier unlocking. For more in-depth information on unlocking, including AT&T's recent allowance of iPhone unlocking for out-of-contract users, see this Ars Technica article.