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The iPhone 4 introduced the concept of multitasking to the Apple iPhone platform, a feature that was absent from previous versions. And now, with the release of iOS 4, the iPhone 3Gs and some iPod Touch devices also enjoy this ability.
The benefit of multitasking is that you don't have to re-open an application every time you want to use it. The downside is that the more open applications you have, the more battery life is drained.
To access open applications on the iPhone 4 and other multitasking enabled Apple devices, all you need to do is tap the Home button two times quickly. This brings up the Multitask Bar which is a list of apps running on your device.
Typically, this is used to quickly access an application you have running on the phone. All you do is tap the icon and the iPhone switches to that application
But what is not immediately obvious is that the Multitask Bar is also used to shut down applications you don't want to leave up and running.
To shut down an application just press and hold any of the open application icons and a minus symbol will appear. Clicking on the symbol will close the application.