The retina is the thin layer of cells that lines the back of the eyeball in humans and in many animals. It is made mainly of neurons and its role is to sense the light which gets into the eye and send information about it to the brain.
Most information about light that leaves the retina travels through the optic nerve, and is used by the brain to recognize what you're seeing. Information about light which travels through the retino-hypothamalic tract is for adjusting circadian rhythms to the 24-hour day. The center of the human retina, called the fovea, has the most rod cells and cone cells for receiving light. Even though the fovea takes up only 1.5 square millimeters, it plays the most important roles in vision. It is only around 1% of the human retina, but the fovea sends about as much information to the visual cortex as the rest of the retina combined.