Bootstrap is an open-source front-end framework for developing responsive and mobile-first websites. It provides a collection of CSS and JavaScript components that facilitate the design of user interfaces, including navigation bars, buttons, forms, modals, and more.

Key features of Bootstrap include: Responsive grid system for creating flexible layouts Pre-styled components like buttons, cards, forms, and navigation bars Customizable using Sass variables and mixins JavaScript plugins for interactive components such as modals, carousels, and dropdowns Cross-browser compatibility

You can customize Bootstrap by: Overriding default styles using custom CSS. Using Bootstraps Sass variables to change the default theme. You can download the source files and modify the _variables.scss file to adjust colors, fonts, spacing, etc. Using Bootstraps built-in utility classes to apply custom styles directly in your HTML.