Eyeshine

Eyeshine or tapetum lucidum is when an animal's eyes glow in the dark.
When light goes into the animal's eye, it hits a web of cells called the tapetum behind the light collector. These cells reflect the light like a mirror. The light goes through the animal's light collector twice, once on the way in and once on the way out. This helps the animal see in the dark.
Most animals with eyeshine are animals with fur, but bullfrogs and some spiders have eyeshine too. 
Different animals' eyes glow different colors: