Bootstrap is a free CSS framework. Bootstrap 3 is the most stable version of Bootstrap, and it is still supported by the team for critical bugfixes and documentation changes.
Bootstrap 4 is a newer version of Bootstrap; with new components, faster stylesheet and more responsiveness. However, Internet Explorer 9 and down is not supported.
Bootstrap 5 is the newest version of Bootstrap; with a smooth overhaul. However, Internet Explorer 11 and down is not supported, and jQuery is replaced with JavaScript.