Say for example, that you and I write a camera program for the iPhone. The only way to get our program on the iPhone is to submit it to Apple and they either approve or deny it. What if they deny it? What do we do?
That is where jailbreaking comes in. Jailbreaking allows you to install software that Apple does not approve of. New features and functionality are now possible.
Turn your iPhone into a WiFi hotspot and surf the internet on your laptop etc. at home or driving down the interstate. Use Facetime over 3G instead of just WiFi. Enjoy the benefits of AirPlay technology without limits. Change your settings such as Brightness, WiFi, and Bluetooth with a swipe of your finger. Even hide unwanted icons.
Jailbreaking is often confused with unlocking. Unlocking means you can use a different carrier such as T-Mobile.