While programming the css styles for a responsive page with Bootstrap 3, you will see that there are 4 types of columns: xs, sm, md and lg. You can set, in the same div, the behaviour for each of this screen sizes.
The best thing to avoid repeating the same is to take the smallest one and this will be applied for the rest.
<div class="col-md-8 col-md-8 col-lg-8"></div>
This will be the same as if you put
Because the sm will be taken as default for all the larger sizes