.visible-**-block elements are displayed as block-level elements on ** devices;
.visible-**-inline elements are displayed as line elements on ** devices;
.visible-**-inline-block elements are displayed on ** Display as an inline block element on devices;
.hidden-** hides on ** devices; (displays on other devices)
.container creates a container for the grid (fixed width);
.container-fluid creates a container for the grid (100% width);
.row means a new row;
.col-**-number defines the column;
.col-**-offset-number shifts the column to the right by the number of columns to add space next to the column;
.col-* *-pull-number the number of columns to move to the left;
.col-**-push-number the number of columns to move to the right;
.clearfix to clear the float;
.well to set the collapse effect on the element;
.close to close the button <button classL :'close' type:'button'>×</button>;
.pull-left float left;
.pull-right float right;
.center-block block-level centering;