Bootstrap ships with 12 grid column system and it prioritize the small screen devices first then up to larger screen devices. The following demonstrate how the twitter bootstrap grid system works. It is one row using various grid system classes comes with it.
Bootstrap's visibility classes
The lg or large grid is now displaying and this grid stacks horizontally at < 1200px.
The md or medium grid is now displaying and this grid stacks horizontally at < 992px.
The sm or small grid is now displaying and this grid stacks horizontally at < 768px.
The xs or extra small grid is now displaying and this should always horizontal.
.col-lg-4 .col-md-1 .col-sm-5 .col-xs-5
.col-lg-4 .col-md-5 .col-sm-1 .col-xs-6
.col-lg-4 .col-md-6 .col-sm-6 .col-xs-1
Bootstrap's grid system classes
Showing col-xs-* grid
.col-xs-5
.col-xs-6
.col-xs-1
Showing col-sm-* grid
.col-sm-5
.col-sm-1
.col-sm-6
Showing col-md-* grid
.col-md-1
.col-md-5
.col-md-6
Showing col-lg-* grid
.col-lg-4
.col-lg-4
.col-lg-4