How to Close Unused Apps on an iPhone
The iPhone 4.0 and later supports multitasking or the use of multiple applications simultaneously on your device. IPhones running the iOS 4.0 or later operating system do not automatically close applications when you exit the application. Therefore, you will need to manually close the unused applications by accessing your iPhone's icon dock bar.
Press the Home button on your iPhone two times in rapid succession. Your iPhone will display the icon dock bar.
Press and hold any icon displayed on your iPhone's dock bar for three seconds until the icons start to move. Release the icon.
Press the upper-left corner of any of the icons to close the unused application on your iPhone. Repeat this step for all unused applications you want to close.