title: how to use bootstrap3 grid system
body: <p>I have been using twitter bootstrap for a year, its the one of those things which -"just works" . With release of twitter bootstrap3 its become even better and awesome with its mobile first approach. But i am not able to understand how to use it properly with mobile first approach.</p>
  
  <p>Earlier there was <code>.span*</code> and <code>.offset*</code> class but now there are <code>.col-xs-* .col-md-* .col-sm-* .col-lg-*</code> and <code>.col-xs-offset-* .col-md-offset-*</code> classes respectively. Even now 'it-just-works' using <code>.col-md-*</code> i want to know the right way to use all these classes so as to not just use them but use them correctly to get most out of bootstrap3.</p>
  
  <p>Thanks.</p>