Google ANDROID Quick Start Guide (4.4, KitKat) - Manual (Page 9)

ANDROID QUICK START GUIDE WELCOME TO ANDROID 5
Some apps hide these buttons temporarily or fade them to small
dots in the same position. To bring them back, touch the dots,
touch the screen in the middle or swipe up from the bottom.
Google Play edition navigation buttons
Some Google Play edition devices have two or three physical
buttons just below the screen:
• Menu: This button opens options relating to the screen you’re
on.
• Home: Opens Home. If you’re viewing a left or right Home
screen, the main Home screen opens. To open Google Now,
touch & hold this button.
• Back: Opens the previous screen that you were working in,
even if it was in a different app. Once you back up to the
Home screen, you can’t go back any further in your history.
Menus
Many screens in apps and Settings include either a Menu
icon (for example, on Nexus devices) or items in the permanent
Menu button at the bottom of the screen (on some Google Play
edition devices). Touch it to explore additional options, including
help.
Organise your Home screens
To change the wallpaper for your Home screens, touch & hold
the background.