In windows 8.1 when you supposedly close an app, it puts it in suspended mode this allows it to be easily resumed when reopening while this can be helpful, it also uses some of your RAM so if you don't want this to happen and you want to fully close an app here's what you do;
1 Move your cursor to the top of the screen
2 click and drag down the window to the button but don't let go of it.
3 Wait until the app board flips and then release it.
The app will now fully close and you don't have to worry about it using memory when you're not using it.