title: HTML/CSS "Pop-Up" Window and Disabled Background
body: <p>This is a silly question since I can't find the right keywords to use to get the answer by searching Google, unfortunately.</p>
  
  <p>You know when you click a link and the background dims and becomes unusable but the foreground either has an image or a sign-in box usually?  Like the Yahoo mail image displaying method where everything in the background becomes grey transparent and the image itself is just fine?</p>
  
  <p>How is that done? And what is it called?</p>