Tailwind CSS is a utility-first CSS framework that enables rapid UI development by composing small, single-purpose classes directly in markup. Unlike traditional CSS frameworks that provide pre-built components, Tailwind gives you low-level utility classes as building blocks—making it infinitely customizable while keeping your CSS lean and maintainable. With version 4's CSS-first configuration and revolutionary JIT (Just-In-Time) compilation, Tailwind generates only the styles you actually use, resulting in microsecond build times and production CSS files often under 10KB. What sets Tailwind apart is its mobile-first responsive design system and state-variant modifiers (hover:, focus:, dark:) that let you handle complex interactions without writing custom CSS—transforming how developers think about styling in modern web applications.