How to switch between your recently used apps on Android
Android is inherently capable of multitasking, and has a sophisticated mechanism for deciding which applications should be kept running in the background, and which should be killed to conserve system resources. That means Android keeps tabs on which applications are running, and when.
It turns out there's a simple way to tap into this mechanism: A long press of the Home hardware button on your device pops up an overlay screen showing the most recently-used applications. You can then just tap an application's icon to instantly switch to it. Handy!