Rods and Cones
In your eyes are various structures. Each has a specific function to help you see. Two of those organelles that respond directly to light are the rods and cones. The cones are responsible for distinguishing color, while the rods distinguish between light and dark. There is also a well documented phenomenon linked to the Headless Horseman related to these structures.
Rods, named after their cylindrical shape, help your brain to distinguish between light and dark. They are concentrated at the outer edges of the retina, so are responsible for peripheral vision. When you see something out of the corner of your eye, you are using your rods to see them. They work very well in low-light conditions, are are the prime source of night vision in humans. They are receptive to a single photon of light, which is almost 100 times more sensitive than the cones. The sensitivity comes at the expense of visual acuity, or the "sharpness" of the image. So while we are allowed to see in lower light conditions with our rods, they give a much more blurry and indistinct image.
Cones, surprisingly, are a conical shape. They are responsible for distinguishing between colors. They are also more able to produce a focused image, and are more receptive to changes in the image than the rods. They are concentrated in the center of the retina, so we use them when staring straight at an object. Cones are much less sensitive to light, needing almost 100 times the amount of light as rods to become activated. So in dark condition, the cones are not stimulated and we cannot see in color as well. This is why it is much more difficult to distinguish between similar colors at night, yet still relatively easy to distinguish between light and dark shades.
The Headless Horseman
Before the advent of streetlights and people yelling out their name to eachother, people would have to peer through the darkness to try to identify oncoming riders. They noticed that the more they stared at the rider, the rider's head would disappear. This was caused the cones' insensitivity to low light conditions. By staring straight at the rider, they were forcing their eyes to use their cones, which were not activated. So their brain would not perceive anything, and the rider's head would disappear. People soon discovered that if they moved their eyes around, they would force their eyes to use their rods, located around the edge of the retina, which were being activated by the low light. By moving their eyes around, they were able to see the rider, albeit a blurry rider.