Ref: http://developer.android.com/guide/topics/resources/index.html
Externalizing your resources also allows you to provide alternative resources that support specific device configurations such as different languages or screen sizes, which becomes increasingly important as more Android-powered devices become available with different configurations. In order to provide compatibility with different configurations, you must organize resources in your project's res/ directory, using various sub-directories that group resources by type and configuration.
For example, while your default UI layout is saved in the res/layout/ directory, you might specify a different UI layout to be used when the screen is in landscape orientation, by saving it in the res/layout-land/ directory. Android automatically applies the appropriate resources by matching the device's current configuration to your resource directory names.
