iOS jailbreaking, or simply jailbreaking, is the process of removing the limitations imposed by Apple on devices running the iOS operating system through use of custom kernels. Such devices include the iPhone, iPod Touch, iPad, and 2nd Gen Apple TV. Jailbreaking allows users to gain root access to the operating system, allowing iOS users to download additional applications, extensions, and themes that are unavailable through the official Apple App Store. A jailbroken iPhone, iPod Touch, or iPad running iOS can still use the App Store, iTunes, and other normal functions, such as making telephone calls. Unlike rooting an Android device, jailbreaking is necessary if the user intends to run software not authorized by Apple. A tethered jailbreak requires that the device be connected to a computer each time it needs to be booted; an untethered jailbreak allows the device to be powered without computer assistance. Under the Digital Millennium Copyright Act, jailbreaking Apple devices is legal in the United States, although Apple has announced that the practice "can void the warranty". Read More at Wikipedia.com
Access to T-Mobile GSM Network
The biggest problem with the iPhone is not the "death grip" that blocks your mobile signal -- it's AT&T. AT&T has exclusive rights over the iPhone until at least 2012, and with all of the network's problems -- mostly related to its tendency to drop calls -- many are justifiably wary of purchasing a phone tied to a problematic network. But the latest jailbreak gives users access to T-Mobile's GSM network, which works overseas and may be more robust in your area.
Cool Jailbroken iPhone Apps
Apple's legit App Store has many cool offerings, but jailbroken iPhones have a wide array of banned creative and quirky apps. Access to Cydia -- the hub of "illegitimate" apps -- is a huge motivation for jailbreaking your iPhone. There are many lists of great jailbreak apps to check out -- here are a few standouts:
MyWi: Turns phones into mobile hotspots
Intelliscreen: Puts e-mail and calendars on the home screen
MxTube: Download YouTube videos onto your phone for offline viewing
My3G: Allows for previously Wi-Fi-only processes to run on 3G, like HD YouTube videos
PDANet: Share Internet connection
Fast and Easy
One of the best parts about jailbreakme is how quick and easy it is to open up your iPhone. No need to download software onto your desktop or hunt for updates that dodge jailbreak-killing iTunes updates -- by using the Web browser, the whole process takes less than a few minutes over a Wi-Fi connection. Still, you can expect Apple to release a patch that will shut jailbreakme down at least temporarily.
If you're scared of the consequences of jailbreaking your iPhone -- or simply don't like the experience -- jailbreakme is fully reversible. According to jailbreakme.com, "If you ever decide that you want to remove your jailbreak, just connect your device to your computer, sync, and then press Restore in iTunes." Beware: if you upgrade past iOS version 4.0.1, there may be some unknown outcomes.
FaceTime Video Chat over 3G
FaceTime video chat is a nifty new feature of the iPhone 4, but it only works over a Wi-Fi network, which may be difficult to find in many situations. Jailbreaking the iPhone 4 allows for FaceTime video chat over a 3G connection. The results are less-than-pretty, but it's more convenient and won't be interrupted if you have a good signal or walk out of Wi-Fi range.