title: "use" keyword/word in Ruby/Rails/Rack code
body: <p>Recently I happened to see this word in Ruby code, <code>use</code>, when I was going through some code related to <a href="https://github.com/postrank-labs/goliath" rel="noreferrer">goliath</a>, middleware etc. Looks like it is different from <code>include</code>/<code>extend</code>, and <code>require</code>.</p>
  
  <p>Can somebody explain why this <code>use</code> keyword exists, and how it is different from <code>include</code>/<code>require</code>? How does it work, when to use it?</p>