The dog's eye is built differently from the human eye. To understand how it works we must explain how the eye perceives light and movement. Inside the eye are cells called rods and cones. The cones are responsible for separating the colors while the rods are responsible for receiving dim light and vision in the dark.
In relation to dogs, humans have many more cones in their eyes, resulting in a higher perceived range of colors that that of the dog. Dogs are like as if they were suffering from a form of color blindness that prevents them from seeing green and red. This can be presented in the pictures below more clearly.
But dogs have a lot more rods in their eyes. This helps them to see better in the dark and to be much more sensitive to movement stimuli than humans. Dogs also have a layer of tissue in their eyes that humans do not have and it is called tapetum lucidum (Latin for bright cover). This reflects light into the retina giving higher brightness in the dark. This is also what we see reflected in their eyes sometimes in the dark.
Also, the position of the eyes is very important. The humans have the eyes positioned to look almost parallel and in front so that there can be a very large common field of vision between the eyes and get a good sense of depth. In dogs the eyes are on the sides, with a small common field of view but a wider range. This makes them have a much better sense of the environment but cannot see well beyond a small distance.