Just like on desktop computers, iPhone programs can sometimes crash and lock up your system. These crashes are much rarer on iPhone and other iOS devices than on computers, but when they do show up, it's important to know how to kill the app--the expression used for quitting apps--that's causing the problem.
Knowing how to kill an app can also be useful because some apps have functions that run in the background that you may want to stop. Quitting those apps completely will cause those functions to stop working.
It's worth noting that there is a popular belief that apps or processes running in the background can drain battery life even when they're not being used. This article used to make that claim. That's since been proven to be incorrect, so I've updated. Check out this article to find out why.
These techniques apply to all devices that run the iOS: the iPhone, iPod touch, and iPad.
Kill the App
In iOS 4 or higher, it's easy to kill the offending app by taking advantage of the built-in Fast App Switcher.
- To access this, double click the home button to reveal the row of apps below the dock
- Slide the apps from side to side to find the one you want to quit
- When you find it, tap and hold the app until a red badge with a line through it appears. The apps will also wiggle like they do when you're rearranging them
- When the red badge appears, tap it to kill the app and any background processes it might be running
- When you're killed all the apps you want, click the home button again to return to using your iPhone.
Force Quit the App
On older versions of the iOS that didn't include multitasking, or when the Fast App Switcher won't work, hold down the home button at the bottom center of the iPhone for about 6 seconds. This should quit the program and return you to the main home screen. If it doesn't, you may need to reset the device.