a new android I just drew. source-code: android_look.svg. CC-BY-SA as usual.

You can acess internal Android resources such strings, drawables, layouts and others. For example, if you need to create a button with the text “Cancel” you can do:

<Button 
   android:id="@+id/CancelButton"
   android:text="@android:string/cancel" 
/>

Using this you are using the internal resource for “Cancel” in that Android and all its i18n. Using the same logic you can access drawables, layouts, etc.