Peripheral vision refers to the vision that lies outside the center of the field of sight, called the fovea. The vision which is near the fovea is called near-peripheral, while that which lies slightly further out is called mid-peripheral. At the very edge of our vision is far-peripheral vision.
When compared to many animals, humans have fairly weak peripheral vision, especially when it comes to distinguishing colors and precise shapes. Our peripheral vision is adapted to be able to understand shapes and forms to get a general impression of a situation. The fovea is far more adapted to distinguish fine detail and color. Even though we are not always consciously aware of this fact, it is easily observable. For example, we know that in order to read printed words on a page, we must trace the narrow center field of our vision back and forth over the text.
The distinctions between central, or foveal, vision and peripheral vision arise from differences in the anatomy of the cells which make up the retina, the part of the eye that receives light and transmits information into the optic nerve. The part of the retina where central vision occurs is heavily packed with cells known as cone cells, which perceive colors and fine lines.
Cone cells make up only a small minority of the retinal cells. The rest are known as rod cells, and these are best for taking in coarser and more general information. Peripheral vision, especially in the far periphery, is accomplished mainly by rod cells. These cells organize light from broad scenes and large objects and convert in into nerve impulses, which reach the brain via the optic nerve at the back of the eye.
The fact that our vision is much more precise and acute at the center of our field of view does not mean that peripheral vision is in any way inferior, just that it accomplishes a different purpose. Indeed, if our whole field of view were as precise as it is at the fovea, the amount of information our eyes sent to our brain would demand much more energy to process. The rod cells which accomplish peripheral vision are also responsible for our ability to see in low-light situations, such as at night. Cones only function to perceive color in well-lit environments. Even though this is a vital function, it would clearly be incomplete on its own.