title: ILookup interface vs IDictionary
body: <p>How does the <a href="http://msdn.microsoft.com/en-us/library/bb534291.aspx" rel="noreferrer"><code>ILookup&lt;key, value&gt;</code></a> interface differ from <a href="http://msdn.microsoft.com/en-us/library/s4ys34ea.aspx" rel="noreferrer"><code>IDictionary&lt;key, value&gt;</code></a>?</p>
  
  <p>I don't understand what the <a href="http://msdn.microsoft.com/en-us/library/bb534291.aspx" rel="noreferrer"><code>ILookup</code></a> interface is meant for.</p>