title: What's the difference between Microsoft.Excel.Office.Core and Microsoft.Office.Interop.Excel?
body: <p>I have to read into creating Add-Ins for Excel and alike and am starting to wonder what the difference between the two namespaces are?</p>
  
  <p>At every tutorial I read, <code>Interop</code> is being used. But in the books, they mostly use <code>Office</code> -- why are there two namespaces, containing more or less the exact same classes, yet are different from each other?</p>
  
  <p>Is there any reference that makes this easy to understand?</p>