According to Microsoft “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”. They might say that, but I’ve seen an app sitting in memory several days later (yes not slowing Windows down but still taking up my precious memory.)
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.
However, to completely stop all processes associated with an application and remove any trace from memory, drag the application to the bottom of the screen and hold it there until the application changes to its default blank screen icon (or as Microsoft says – “it flips over”).