Bootstrap is the most popular, open source website starter toolkit, developed by the team at Twitter. It contains predesigned, responsive frontend components which you can use as a basis for creating web sites or web applications. For example, it includes a nice grid system for the layout, alerts, buttons, carousels, tooltips, to name a few.
It makes the designing simpler, since it provides some nice-looking default styling for standard html elements as well. So we don’t have to write our CSS stylings from scratch. We only have to include Bootstrap files and use the correct class names for our HTML elements and they will be styled automatically through Bootstrap.