How to Kill an App in iOS 4
The iPhone iOS 4 allows any application that supports multitasking to automatically run in the background. If you open a lot of apps, they can drain your battery and slow down the iPhone's performance. You can kill these background apps at any time using the phone's Quick Switch menu. Killing the apps you are not using can speed up the phone's performance and increase the battery life.
Double-tap on the "Home" button (the physical button on the face of the iPhone) to bring up the Quick Switch menu.
Tap and hold on the icon of the app you want to kill. Hold the tap until a red "-" symbol appears in the top left corner of the app icon.
Tap the red "-" to kill the app.
Tap the "Home" button to exit the Quick Switch menu.