Long press back button like u have already done before, this shrinks the apps, like u mentioned, and shows u all apps that are open (I say open, but actually that's not true. All apps u see that appear to be open are actually suspended in the background) You can swipe left or right to pick an app to resume from where u left off, and now all other apps are suspended, this is multitasking in WP OS. To close an app, you should see a X in the top corner of each app once you long press the back key. If you do not see this option, then you need to update your phone to the latest version of WP8, just search the forums if you dont know how or need help figuring out which version of WP your running.