In Android, once you launch an application, it will continue to run in the background, even after you’ve left it and moved on to other tasks. After enough applications are opened in a session, you may notice your device lagging a bit due to the number of open applications using up your device’s RAM. This isn’t so much of a problem on newer, highly-specced devices like the Galaxy S4, but several less powerful devices still stutter a bit when too many applications are open and running. Also, if someone else picks up your phone, they will also be able to see what apps you've been using which is a privacy issue.
Here’s how to kill applications running in the background.
1. Launch the recent applications menu. How you get to this menu will vary a bit from device to device. On the HTC One, for example, you tap the home button twice to get to Recent Apps, whereas it takes a long press on the Samsung Galaxy S4’s physical button, and an on-screen button takes you to this location on the Nexus 5.
2. Find the application(s) you want to close on the list by scrolling up from the bottom.
3. Tap and hold on the application and swipe it to the right. This should kill the process from running and free up some RAM.
4. Navigate to the Apps tab in settings if your phone’s still running slow. On Samsung phones, this menu is called "Application Manager."
5. Select the Running tab to see what's still running.
6. Return to the "All apps" tab, scroll to the application(s) that’s running, and click on it.
7. Tap "Force Stop" to kill the process for good.