Tailwind CSS is an open-source, utility-first CSS framework that enables developers to build custom designs directly within their HTML. Unlike traditional CSS frameworks that offer predefined components, Tailwind provides low-level utility classes that can be composed to create any design, offering greater flexibility and control over the styling process.

Key features include:

Utility-First Approach: Build designs by combining utility classes like 'flex', 'pt-4', 'text-center', and 'rotate-90' directly in your markup.
Responsive Design: Easily apply styles at specific breakpoints using screen size prefixes such as 'sm', 'md', 'lg', and 'xl'.
Dark Mode Support: Implement dark mode by prefixing classes with 'dark:', for theme switching.
CSS Grid and Flexbox: Utilize modern layout techniques with built-in utilities for CSS Grid and Flexbox.
Customizable Design Tokens: Define colors, spacing, and typography centrally using CSS variables, facilitating consistent theming across your project.

Tailwind CSS is free to use under the MIT license, making it accessible for both personal and commercial projects. It runs in the web browser, allowing developers to integrate it into their projects without additional software requirements.