This article will explain the Multitasking feature in iOS 4, and how to use it.
Multitasking is a feature of iOS 4 that allows applications to perform specific tasks in the background while you are using other applications. Multitasking in iOS 4 is supported on iPhone 4, iPhone 3GS, and iPod touch (3rd generation). Shutting down these applications can help alleviate strain on system memory on some older devices, or on devices with numerous multi-tasking apps running on startup. To temporarily shut down an application running in the background on your device, you can follow these steps:
1) From the Home screen on your device, double-tap on the Main button for your device.
2) Once the Multitasking window comes up, tap and hold any applications other than the game you're playing until a red minus sign appears.
3) Tap on the minus symbol to quit the application. This also removes the application from the list of recently used apps.
Any application you remove by this method will run in the background again when you restart your device, or when you launch the application.
For further information regarding the Multitasking feature, please refer to the following article on Apple Inc's support database: