The light detecting organ in the eye is the 'retina' which lines the inside of the eye and is covered in special receptors. These receptors absorb the light that hits and then generate an electrical impulse which travels down the 'optic nerve' to the brain. The brain receives thousands of these small messages every millisecond that you eyes are open and it uses them to build up the constantly changing image of the world that you live from. There are two sorts of receptors in the mammal eye. They are called 'rods' and 'cones'.
Rods are better for night vision because they are sensitive to small quantities of light.
Some nocturnal animals such as opossums have only rods in their retina which gives them excellent night vision. However as it is the cones that allow us to distinguish between different colours of light they miss out on a lot of the beauty of the world. Also there can be as many as 150 rods connected to a single nerve cell, this increases its sensitivity but reduces its definition, i.e. things look blurry. Cones can be as few as one or two per nerve cell, this works the opposite way making them less sensitive but more accurate for detail. Which is good during the daylight.