A Hack developer works with Facebook's programming language, Hack, which is designed for the HipHop Virtual Machine (HHVM). They create and maintain web applications, ensuring efficient functionality and optimal performance. Their duties include writing clean, testable code, debugging programs, integrating user-facing elements, and developing back-end components. They collaborate with front-end developers, designers, and system admins to produce high-quality web applications. Hack developers also design and implement data storage solutions, ensure security compliance, and upgrade applications to improve user experience. They stay updated with industry developments to maintain proficiency in Hack programming.