Double-tapping the Home butons brings up the
Recently Used Apps or the
Recents List
See the iOS4 Manual:
And from
iOS 4: About multitasking
From
Apple's webpage on multitasking:
The Apps shown in the *Recently Used Apps* list, are NOT all running in the background.
Only Apps that have been specifically updated for iOS 4 compatibility to use one of the new multitasking services (Voice over IP, background audio, background location) can be running in the background. Any others are simply frozen in memory using iOS 4's Fast App Switching.
Apps that have not been updated for iOS 4 cannot run in the background AT ALL. They are simply listed in the *Recently Used Apps* list, because you have used them recently. They do not need to be removed or shut down.