Image Source :- http://getbootstrap.com/
Image Source :- http://getbootstrap.com/

What is Bootstrap ?
Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web. Bootstrap is a free collection of tools for creating websites and web applications. It contains HTML and CSS-based design templates for typography, forms, buttons, navigation and other interface components, as well as optional JavaScript extensions. In June 2014 it was the No.1 project on GitHub with 69,000+ stars and 25,000+ forks(Source: Wikipedia).

How to download and use Bootstrap?
To download the latest Bootstrap available visit http://getbootstrap.com/, after that unzip the compressed folder. It usually contains three folders inside it namely css, js and fonts. The folder includes the following content :

bootstrap/
├── css/
│   ├── bootstrap.css
│   ├── bootstrap.min.css
│   ├── bootstrap-theme.css
│   └── bootstrap-theme.min.css
├── js/
│   ├── bootstrap.js
│   └── bootstrap.min.js
└── fonts/
    ├── glyphicons-halflings-regular.eot
    ├── glyphicons-halflings-regular.svg
    ├── glyphicons-halflings-regular.ttf
    └── glyphicons-halflings-regular.woff

(Source: www.getbootstrap.com)

To create your first bootstrap website, include the above files in the head of your HTML document.

A sample Bootstrap HTML document:
Here is what a normal Bootstrap HTML document looks like.

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Bootstrap 101 Template</title>

<!-- Bootstrap -->
<link href="css/bootstrap.min.css" rel="stylesheet">

<!-- HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries -->
<!-- WARNING: Respond.js doesn't work if you view the page via file:// -->
<!--[if lt IE 9]>
https://oss.maxcdn.com/html5shiv/3.7.2/html5shiv.min.js
https://oss.maxcdn.com/respond/1.4.2/respond.min.js
<![endif]-->
</head>
<body>
<h1>Hello, world!</h1>

<!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js
<!-- Include all compiled plugins (below), or include individual files as needed -->
http://js/bootstrap.min.js
</body>
</html>

(Source: www.getbootstrap.com)