title: What does (.:format) refer to in Rails and how does it interact with the _path and _url helpers?
body: <p>I've been writing my routes manually but gave up after realizing how pointless that would be in Rails given that a large part of the purpose of this framework is automating repetition. </p>
  
  <p>After generating my routes using Rails' <code>resources</code> keyword, I'm unable to find any explicit documentation explaining what the <code>(.:format)</code> parameter represents. I'm also unclear on how <code>_path</code> and <code>_url</code> helpers engage these routes: </p>
  
  <pre><code>posts     GET    /posts(.:format)              posts#index
            POST   /posts(.:format) posts#create
  new_post  GET    /posts/new(.:format)       posts#new
  edit_post GET    /posts/:id/edit(.:format) posts#edit
  post      GET    /posts/:id(.:format)           posts#show
            PATCH  /posts/:id(.:format)            posts#update
            PUT    /posts/:id(.:format)            posts#update
            DELETE /posts/:id(.:format)            posts#destroy
  </code></pre>