title: What does .:format mean in rake routes
body: <p>I type rake routes and I get a bunch of urls like this - <code>/articles/:id(.:format)</code></p>
  
  <p>My question is - what does the <code>.:format</code> mean? It is not clear from the Rails Guides Routing article and there are no other helpful matches for <code>.:format</code> on StackOverflow or google. There is a similar format which is <code>/:controller(/:action(/:id(.:format)))</code> which I also don't understand. </p>
  
  <p>Thanks</p>
  
  <p>EDIT follow up question - </p>
  
  <p>If  I wanted to only route HTML pages. Would it be best practice to specify something like .:html in the route or to use .:format and just write a respond_to block for format.html? Would all other formats be ignored in that latter case? </p>