Gatsby JS is a free and open-source framework based on React that helps developers build fast and secure static websites and apps. It provides an ecosystem for creating dynamic websites with modern web technologies and is known for its speed, scalability, and a growing plugin system to integrate third-party services. While working with Gatsby JS, developers use GraphQL to source data from various APIs and static markdown files. The framework is optimized for performance and is suitable for building modern web projects, ranging from personal blogs to complex websites.