title: password_hash/password_salt vs password_digest
body: <p>I'm currently going through Michael Hartl's book (Ruby on Rails tutorial 3rd edition) and he uses <code>password_digest</code> with his authentication model, I checked out Pragmatic Studio's Rails course and in their sample video, they use <code>passowrd_hash/password_salt</code> in their authentication model. From wikipediaing, it seems that salt is a standard use in cryptography and has a variety of uses for security. All I could really find on password_digest is it works with a rails method <code>has_secure_password</code>.</p>
  
  <p>Is one preferred over the other or do they serve different purposes ultimately? </p>