A pixel shader is a type of computer program used in graphics processing to determine the color and brightness of individual pixels on a screen. It operates within the graphics pipeline, taking input data such as textures and lighting information to create realistic images in video games and other visual applications. These shaders allow for complex visual effects, such as shadows, reflections, and surface textures, enhancing the overall appearance of 3D models. By manipulating pixel data, pixel shaders contribute significantly to the realism and artistic style of graphics in software like OpenGL and DirectX.