title: How does the Android source overlays work?
body: <p>In the android source in the <a href="http://android.git.kernel.org/?p=device/sample.git;a=tree;f=overlays/backup;h=3bc090038cb5af78365f6d4584d40a1c89a52e9a;hb=HEAD" rel="noreferrer"><code>device/sample</code></a> folder there is a folder called <code>overlays</code>. You see the same overlay folder in e.g. the <a href="https://github.com/CyanogenMod/android_device_htc_hero" rel="noreferrer">cyanogen mods</a>.</p>
  
  <p>What is this overlay folder? How does it work? What is it used for? Where can I read more about it?</p>
  
  <p>Thanks in advance</p>