For questions relating to the recent apps list. Older devices access this list by long pressing the home button, whereas newer devices (without hardware buttons) have a dedicated button on the right of the button bar.
The recent apps list provides a thumbnail view of recently opened apps, allowing for easier multitasking on Android devices.
On devices with hardware buttons, the recent apps list is opened by long pressing the home button. Devices that lack hardware buttons (such as the Galaxy Nexus or Motorola Xoom) will instead have a dedicated button on the button bar that looks like two overlapping frames.