Bootstrap is the framework in which responsive designs can be made. Responsive design means which can be fit to any device of any resolution

Bootstrap is a free front-end framework for faster and easier web development

Bootstrap includes HTML and CSS based design templates for forms, buttons, tables, image and many other, as well as optional JavaScript plugins

Bootstrap also gives you the ability to easily create responsive designs

To include bootstrap

 <!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<!-- jQuery library -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>

<!-- Latest compiled JavaScript -->
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>