A cat's vision and a human's vision differ considerably. One difference between human vision and cat vision lies within the retina. The retina is a tissue layer containing photoreceptor cells located at the back of the eye. These photoreceptors are responsible for the conversion of light rays into electrical signals which are then processed by nerve cells to the brain and are thus translated into images.
Rods & Cones
There are two types of photoreceptor cells on the retina called rods and cones because of their shapes. Night vision, peripheral vision and the detection of brightness and shades of gray are the job of the rods. Cones are the day vision and color perception photoreceptors.
As you can imagine, cats (and dogs also) have a very high concentration of rod receptors but the opposite is true when it comes to the cone department. We humans have more cone receptors which translates into us seeing colors better but we can't see very well at night! Furthermore, cats have a slightly better visual field than us humans. The visual field is the area the eyes that focus upon a single point. In other words cats can see better straight ahead, above, below and to the side. (The visual field of cats is 200 degrees; humans, 180 degrees.)
Less Color, Better Night Vision
But what about color vision? Is it true cats see only shades of gray? It is a common misconception that cats cannot see any colors. Both humans and cats are 'trichromatic' but not in a totally similar fashion. Humans see red, green and blue (RGB) but those who are color blind see shades of blue and green while red, orange, pink, and browns become washed out. Reds and pinks can look more green and purple can look like a shade of blue.
Cats can see a few colors and can discern between red, blue and yellow. They are also able to distinguish between blues and violets much better than the colors in between, towards the red end of the spectrum. Cats also do not share with us the richness of hues and color saturation we experience, but cats do gain a superiority when night falls because they can see extremely better than we can see in the dark.
UV Light Detection
Basically cats are predators and they need to be able to sense movement in low light situations, so some of their color perception has been sacrificed to accommodate their predatory lifestyle. And cat vision has an added bonus over human vision - cats can see things that appear invisible to us! The secret, as reported in a paper published in the Proceedings of the Royal Society B, found cats, dogs and other animals possess ultraviolet light sensitivity. What this means, for example, is that any animal possessing UV sensitivity can see the patterns on flowers indicating nectar location or urine trails leading to prey. A white furry rabbit hopping though a snowstorm could be seen also, while to a human it would appear as a white blur. This is a pretty fine addition to a cat's vision but for myself, I'm okay living without UV sensitivity... I'd rather not see all the urine markings around our neighborhood, not to mention other places humans frequent!