Do not panic, this is not irreversible. We will immediately see together how to get rid or just totally stop applications running in the background and which are rowing your smartphone.
Stop applications running in the background
If you closed all open applications via the multitasking button in the navigation bar, many of the "process" of different applications that run almost continuously without you knowing. The first thing to do is identify them. To do this, go into the settings and options in developers (follow the link for how to display).
Then find the "Running Services" to see which applications are right now in the process of consuming the RAM of your smartphone or tablet while all applications are yet "closed". Here you will find the system and processes related to certain Google applications we advise you not to close to ensure the proper functioning of your device.
But other applications you use little or not at all may remain in standby for different reasons. You can see in detail the amount of RAM that they use in real time. Now that you have identified your target, it's time to act!
What to do, ask yourself if you really need these applications that consume resources. If this is not the case, go to the Applications tab that is in the settings and uninstall them. If the system does not allow it (and you do not have root access), you can "turn off" the application. Attention will reactivate it if you open it.
Use Greenify to "freeze" applications
Unfortunately, it is often the applications that are used most consuming nonetheless RAM and autonomy when closed. To these, I advise you to download Greenify excellent application that will offer you to "Hyberner" selected applications when you are not using them. A good alternative to uninstalling and disabling.
The application is available free on the Play Store, but there is a paid version that will even further if you have root access, with automatic hibernation system. If you have other techniques to drive energy-intensive applications, share them with the community.