A Smooth Surface and a Thin Layer of Metal
A mirror does not glow on its own. It cannot produce light, and it does not store and release it. What it does is reflect — and it does that very well because of how it is built. Most household mirrors are simply a sheet of glass with a thin coating of metal, usually aluminium, applied to the back. The glass protects the metal and keeps the surface flat. The shiny metal layer is the part that actually does the reflecting.
The Law of Reflection
Light travels in straight lines. When a ray of light hits a mirror, it bounces off again according to a simple rule physicists call the law of reflection: the angle at which the light hits the surface equals the angle at which it leaves. If light strikes the mirror at a shallow angle, it bounces off at the same shallow angle on the other side. This rule applies to every single ray of light that meets the mirror, no matter what direction it came from.
Why a Picture Forms
The reason you see an image and not just a bright glare comes down to one word: smoothness. A mirror is so flat at the level light actually sees it that the rays bouncing off it stay in formation. The light that came from your nose lines up neatly when it leaves the mirror, the light from your eyes does too, and so on. Your eyes catch all those organized rays and reconstruct a coherent picture of what is in front of the mirror. It is essentially the same light, traveling along a slightly different path.
What Happens on a Rough Surface
Any other surface that catches the same light handles it very differently. A wall, a piece of paper, the back of your hand — none of them give a clear image, even though plenty of light bounces off them. Up close, these surfaces are not smooth at all. They are full of microscopic bumps and pits that point in every direction. The law of reflection still applies to every ray, but each ray hits a tiny patch of surface tilted slightly differently, so they leave in scattered directions. The result is that you see the object, lit by reflected light, but not a clear picture of what was in front of it. Smoothness is what separates a mirror from everything else.
Source
This article was written using information from Wikipedia.