While Antiphon's answer is correct, here's a slightly more understandable explanation:
Since glass is a dielectric material, the amount of light that it reflects actually changes depending on the angle of the light striking the glass, unlike metals where the amount of light reflection is constant no matter what angle light strikes the surface. But this aspect of glass means that it can actually achieve perfect 100% reflection, whereas no mirror made with metal can achieve that because there is always some level of absorption (this is why most metals have a sort of greyish look to them).
So how do you get glass to act as a perfect mirror? It's called total internal reflection, and it's how fiber optics work. Look it up, it's got some pretty neat consequences and uses. Basically, when light exits glass into a medium like air with a lower index of refraction, there is a certain angle, called the critical angle, where the angle of the refracted light bends so far back toward the glass that it actually never even enters the air and instead just refracts right back into the glass. This is total internal reflection. Since it's 100%, fiber optics can send information encoded in light for long distances without any appreciable attenuation.
Here's an image to give an idea of what's going on:
https://chemicalparadigms.wikispaces.com/file/view/tir.JPG/33653365/tir.JPG
The above image might lead you to believe that the light source must be inside the glass, but it actually doesn't have to, it just has to pass through the glass so that the angle it makes with the glass to air interface is greater than the critical angle.