title: What is concrete implementation?
body: <p>I saw lot of posts in StackOverflow and elsewhere talking about concrete implementation. While fiddling with WCF i came through the line</p>
  
  <blockquote>
    <p>Tying your service implementation, or
    any “service based” class to a
    concrete implementation is never a
    good idea.</p>
  </blockquote>
  
  <p>Can anyone explain what Concrete Implementation is?</p>