Bootstrap is a free front-end framework for faster and easier web development Bootstrap includes HTML and CSS based design templates for typography, forms, buttons, tables, navigation, modals, image carousels and many other, as well as optional JavaScript plugins Bootstrap also gives you the ability to easily create responsive designs
Bootstrap CDN
< link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css"> < script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"> < script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"> < script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js">

2. Container

Bootstrap 4 also requires a containing element to wrap site contents. There are two container classes to choose from:
The .container class provides a responsive fixed width container
The .container-fluid class provides a full width container, spanning the entire width of the viewport
div class="container">
< h1>My First Bootstrap Page< /h1>
< p>This is some text.< /p>
< /div>

3. Grid System

Bootstrap's grid system is built with flexbox and allows up to 12 columns across the page. If you do not want to use all 12 columns individually, you can group the columns together to create wider columns:
span 1 span 1 span 1 span 1 span 1 span 1 span 1 span 1 span 1 span 1 span 1 span 1
span 4 span 4 span 4
span 4 span 8
span 6 span 6
span 12
The grid system is responsive, and the columns will re-arrange automatically depending on the screen size. Make sure that the sum adds up to 12 or fewer (it is not required that you use all 12 available columns).
Grid Classes
The Bootstrap 4 grid system has five classes:

• .col- (extra small devices - screen width less than 576px)
• .col-sm- (small devices - screen width equal to or greater than 576px)
• .col-md- (medium devices - screen width equal to or greater than 768px)
• .col-lg- (large devices - screen width equal to or greater than 992px)
• .col-xl- (xlarge devices - screen width equal to or greater than 1200px)
The classes above can be combined to create more dynamic and flexible layouts.

Tip: Each class scales up, so if you wish to set the same widths for sm and md, you only need to specify sm.