Android 4.0 removes the need for traditional hardware keys on phones by replacing them with a virtual navigation bar that houses the Back, Home and Recents buttons. Read the Compatibility pattern to learn how the OS adapts to phones with hardware buttons and how pre-Android 3.0 apps that rely on menu keys are supported.
The action bar is the most important structural element of an Android app. It provides consistent navigation across the platform and allows your app to surface actions.
Creating apps that scale well across different form factors and screen sizes is important in the Android world. Multi-pane layouts allow you to combine different activities that show separately on smaller devices into richer compound views for tablets.
The long press gesture which was traditionally used to show contextual actions for objects is now used for data selection. When selecting data, contextual action bars allow you to surface actions.