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.

Earlier there was .span* and .offset* class but now there are .col-xs-* .col-md-* .col-sm-* .col-lg-* and .col-xs-offset-* .col-md-offset-* classes respectively. Even now 'it-just-works' using .col-md-* 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.

Thanks.