title: what is the difference between overlay map type and layers
body: <p>From the google map v3 javascript api,I found this:</p>
  
  <blockquote>
    <p><strong>Overlay Map Types</strong></p>
    
    <p>Some map types are designed to work on top of existing map types. Such
    map types may have transparent layers indicating points of interest,
    or showing additional data to the user. (Google's traffic layer is an
    example of such a map type.)</p>
  </blockquote>
  
  <p>Which say that <strong>Google's traffic layer is an example of such a map type</strong>.</p>
  
  <p>But the <a href="http://code.google.com/apis/maps/documentation/javascript/layers.html#TrafficLayer" rel="nofollow">traffic layer</a> is intance of Layer in google map.</p>
  
  <p>So I am confused with the concept.</p>
  
  <p>Since we are build our own map library for offline use,so I want to refer google's concept and their idea.</p>
  
  <p>Any one can give me more details?</p>