We know that intersection of vertical and horizontal lines create grids. Bootstrap’s grid system divides the screen into multiple rows (horizontal lines) and columns (vertical line). Grid system is the most important feature of bootstrap and it's biggest advantage is that, it is responsive unlike HTML tables, means it is flexible and adjusts itself in all screens because of this feature it is used for responsive web design. The default Bootstrap grid divides the screen into 12 columns in each row. You can add CSS classes to block elements in the HTML document by using bootstrap grid system.