How to Close the Active Apps on an iPhone
The iPhone 4.0 and later supports multitasking. Multitasking allows you to have multiple applications open simultaneously on your device. However, applications no longer will automatically close on your iPhone when you exit the application. Therefore, to free memory on your iPhone, you will need to periodically manually close the active applications on your device.
Press the "Home" button on your iPhone two times. This will activate the multitask bar at the bottom of your iPhone's display. The icons displayed on this multitask bar are the active apps open on your iPhone.
Press and hold any of the icons on the multitask bar. Release the selected icon once all of the icons start to shake.
Tap the "Delete" symbol in the upper left corner of the icon for the active application you want to close. Repeat this step for all active applications that you want to close on your iPhone.