It took me a while to figure out how to close running apps in ios.
the new multitasking feature looks great, but it doesnt behave the same way as ios6 and earlier.
in ios6 and earlier you hold the icon till it wiggles and then hit the X to close the running app.
in ios7 you swipe the screenshot of the app towards the top of the screen to close the instance of that running app.
Cheers was trying to figure the same thing out !!
you can also close multiple apps at once (try swiping 3 apps upwards – it takes a bit of getting used to, you need to use one finger for each app, but it works :)