The retina is the sensory membrane that lines the inner surface of the back of the eyeball. It's composed of several layers, including one that contains specialized cells called photoreceptors.
There are two types of photoreceptor cells in the human eye — rods and cones.
Rod photoreceptors detect motion, provide black-and-white vision and function well in low light. Cones are responsible for central vision and color vision and perform best in medium and bright light.
Rods are located throughout the retina; cones are concentrated in a small central area of the retina called the macula.