A GLSL (OpenGL Shading Language) developer writes code for graphics hardware to render scenes. They create shaders, small programs that run on GPUs, to control pixel color, position, and depth. Shaders are crucial for effects such as lighting, shadows, reflections, and post-processing. GLSL developers often work closely with game developers or in the field of scientific visualization. They need a deep understanding of computer graphics, mathematics, and optimization techniques to write efficient and effective shaders. They also debug and optimize GLSL code to ensure smooth and visually impressive graphics performance.