Apps in the Windows Store are built so they don't slow down your PC when you're not using them. When you stop using an application, Windows leaves it running in the background and then, after a while, closes it for you.
But if you want to close an application, drag it from the top of the screen to the very bottom. (If you're using a mouse, move your mouse pointer to the top of the screen and then click the Close button in the title bar.)
If you can't see the Close button in the title bar, you don't have the latest update to Windows 8.1
and Windows RT 8.1. For more information, see Installing the latest Windows 8.1 Update.
To completely stop all processes associated with an application, drag the application to the bottom of the screen and hold it there until the application flips over.
It's still a good idea to close desktop applications when you've finished using them, particularly before shutting down your PC.