CSS is a style sheet language used for describing the presentation of a document written in HTML or XML. It controls the layout, colors, fonts, and overall visual appearance of web pages. CSS allows developers to separate content (HTML) from design, making it easier to maintain and update websites.
Bootstrap is a free and open-source front-end framework used for developing responsive and mobile-first websites. It includes HTML, CSS, and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.