Once you open an app on an iOS device, it will continue to run in the background unless it is closed manually. Here is an easy way to close an app on an iPad or iPhone:
1. Click the home button twice and a bar containing all of your open apps will appear at the bottom
2. Tap and hold one of the app icons until they all begin to wiggle
3. Tap the red minus icon in the top left corner of an app icon to close it
For more information on managing apps in iOS, please see Apple’s support page.