Apple retains complete control over the App Store in its iOS. If a developer creates an app that breaks Apple's rules, it will not be allowed in the marketplace, no matter how innovative and useful it may be. A jailbroken iPhone can run unauthorized code without restrictions.
Top 10 Reasons NOT to Jailbreak & Reasons to Jailbreak an iPhone!
Apple locks down the iPhone iOS from hackers, but every now and then they find a way to jailbreak the iPhone. It’s harder to jailbreak an iPhone than it used to be. A full jailbreak is rare, but every now and then a bug in an iOS software update makes it possible again.
Is Jailbreaking Worth It? Here are some things to consider before you decide to jailbreak your iPhone.
Developers who can’t sell their software in the official app store have a new market. They use the Cydia app store, which is part of the jailbreaking process. These alternative outlets provide a home for the games and utilities that Apple has rejected.