/System/Library/CoreServices/SpringBoard.app

The SpringBoard is the First thing you see when you power up your iPhone or iPod Touch and pass the lock screen. The SpringBoard has access to all your apps apart from SpringBoard extensions. An app includes but us not limited to: Facebook, TapTap, Quake, Mail and Google Maps etc. You can reorganise apps on the iPhone SpringBoard by tapping and holding. On any app. You can then drag the apps around with your finger. Alternatively, iTunes 9 has an organiser that allows you to do the same on you PC/Mac

And when its features have been exhausted.....

The SpringBoard can be extended to do a whole lot more if you Jailbreak and use Mobile Substrate