You'll probably finish using the Stock application and you press Home button to go back to home screen. You may think that you already quit the application. But not really…

The fact is the Stock application does not actually quit and it’s just “hidden” somewhere in background. So, next time, you launch the Stock application again, iPhone will just unhide it and put it back to foreground. This can certainly minimize the application load time for subsequent launch. But there is always a trade off. As the application is hidden in background, it takes up certain amount of iPhone memory. So, sometimes you probably like to free up more resourcesfor your iPhone by explicitly quit those unused applications.

Force Quit

Normally, you can force quit an iPhone application by pressing and holding down the HOME button for around 6-8 seconds in firmware 2.2 or lower. For iPhone 3.0, to explicitly quit an iPhone app, you should first press and hold SLEEP/POWER button until the power off slider shown. Then you can press and hold HOME button for a while (around 6-8 seconds) to quit the iPhone app. And, it will automatically bring you back to the home screen.