title: In GitHub URL's: what is the difference between a tree and a blob?
body: <p>I keep noticing this whenever I'm trying to add links to github files. You can add a link using <code>blob</code>:</p>
  
  <p><a href="https://github.com/facebook/pop/blob/master/Podfile" rel="noreferrer">https://github.com/facebook/pop/blob/master/Podfile</a></p>
  
  <p>And the same doc comes up if you change blob to <code>tree</code>:</p>
  
  <p><a href="https://github.com/facebook/pop/tree/master/Podfile" rel="noreferrer">https://github.com/facebook/pop/tree/master/Podfile</a></p>
  
  <p>What is the difference? Whenever I want to add a link for posterity in a doc, which one should I prefer?</p>