When exiting an Application it doesn’t always stop the process that the application is doing. For example, when exiting the Music App, if you’re currently listening to music it will keep playing this music. Some Applications even have the ability to sync data after exiting.
These types of processes can drain battery life without you know. To ensure that all of the applications are closed double press the home button and bring up with multi-task tray. Once you’ve done that, tap and hold on an App icon until they start to jiggle and then you can press the red minus circle and it will close the App for good thus saving your battery as well.