jailbreaking an iphone or ipod touch allows you to install applications on it that are not part of the itunes app store. Apple frowns on this. But there are apps such as Google Voice Mobile that the only way you can get them is on a jailbroken iphone or ipod touch, since Apple kicked it out of the App Store. Eventually, Apple may be forced to open up the iPhone and iPod Touch, but that isn’t the story today.