Why do animal's eyes glow in the dark?

So why do they? I have always wondered but I have never looked it up. This is a special feature for nocturnal creatures apparently.

So check out this NPR science segment by Andrea Seabrook. However, I feel an analogy would have helped :

"A lot of the animals we see, especially the ones that go out at night, have a special, reflective surface right behind their retinas," says Dr. Cynthia Powell, a veterinary ophthalmologist at Colorado State University. That light-reflecting surface, called the tapetum lucidum, helps animals see better in the dark.
When light enters the eye, it's supposed to hit a photoreceptor that transmits the information to the brain, Powell explains. But sometimes the light doesn't hit the photoreceptor, so the tapetum lucidum acts as a mirror to bounce it back for a second chance.

" INSERT ANALOGY HERE FOR GREATER CLARITY -- WHAT WOULD HAPPEN IF IT DIDN'T etc. etc. "