A few apps including some GPS apps continue to use full system resources even if you have exited from them, this means they can quickly drain battery power. Sometimes an app might become unresponsive or even crash. In both these instances it is a good idea to fully shut them down.
Press the Home button twice and the app switcher will open showing large thumbnail previews of all your recently opened apps. Swipe left or right until you find the large thumbnail preview of the app you want to close. When it is centered on the middle of the screen simply swipe up.