Bootstrap 3 introduces a new grid system, with four tier of grid classes (phones, tablets, desktops and large desktops). This allows us to create flexible and awesome layouts for each of our target devices. In this tutorial, we will be looking at: Understanding Bootstrap 3 Grid System.
Bootstrap 3 introduces the responsive mobile first fluid grid system that appropriately scales up to 12 columns as the device or viewport size increases.
Bootstrap 3 Grid System
Bootstrap 3 Grid System Extra small devices
Phones (<768px) Small devices
Tablets (≥768px) Medium devices
Desktops (≥992px) Large devices
Desktops (≥1200px)
Max container width None (auto) 750px 970px 1170px
Class prefix .col-xs- .col-sm- .col-md- .col-lg-
Max column width Auto ~62px ~81px ~97px
Gutter width 15px on each side of a column (i.e. 30px)

As you can see from above, elements having a class of .col-sm- will not only affect its styling on small devices (tablets) but also on medium and large devices having screen size greater than or equal to 768px if .col-md- and .col-lg- class is not added to that element. So also, .col-md- class will not only affect the styling of elements on medium devices but also on large devices if .col-lg- class is not added to the element.
Rules of Using Bootstrap 3 Grid System
You must place all rows within a .container class (for fixed-width) or .container-fluid class (for full-width) for proper alignment and padding.
Use rows to create horizontal groups of columns by adding .row class to the element.
Columns must be placed within rows and must have class like .col-md-5.
Content should be placed within columns.

The code below shows how the grid system is used.

<div class="container">
<div class="row">
<div class="col-md-6">

Column One

</div>
<!-- end of .column 1 -->
<div class="col-md-6">

Column Two

</div>
<!-- end of .column 2 -->

</div>
<!-- end of .row -->

</div>
<!-- end of .container -->

Thanks for reading, I hope this tutorial was helpful. If you are new to Twitter Bootstrap, you should read my other tutorials on Twitter Bootstrap:
Introduction To Twitter Bootstrap 3
How To Create A Website With Bootstrap 3
source: http://tutorialslodge.com/understanding-bootstrap-3-grid-system/