Bootstrap is a free, open-source front-end development framework for the creation of websites and web apps. It is designed to enable responsive development of mobile-first websites. Bootstrap provides a collection of syntax for template designs, including pre-defined classes that make a developers job easy. Bootstrap is built on HTML, CSS, and JavaScript. It is the most popular CSS framework for developing responsive and mobile-first websites. Bootstrap saves developers time by providing a set of style definitions across all HTML code. Bootstrap is not a programming language, but a toolkit designed to simplify and streamline front-end development. Developers need to have a basic understanding of HTML, CSS, and JavaScript to work with Bootstrap. Bootstrap has evolved over the years, with four versions till date. The newest version is Bootstrap 5.