How to Close Down Running Apps in an iPhone
The iPhone 4 allows you to multitask, giving you the ability to run several applications simultaneously. As with any device, there's a drawback to leaving too many programs running at once. If you begin to notice significant slowdown while operating your iPhone, close some applications to get your device running normally. Use the phone's multitasking menu to manage and close applications.
Double-tap the home button. Your running applications appear on the bottom of the screen.
Hold your finger down on any application. The application icons start wiggling and a red "-" symbol appears on each icon.
Tap the red "-" symbol. The application closes.