The Law of Light Reflection states that the angle of light incidence is equal to the angle of light reflection. Reflected light rays from various parts of your face can be traced to the reflective surface of the mirror and then back to your eye. Using these "ray diagrams" the location of the image you see is always behind the mirror.

If you allow your eyes to focus on the front of the glass, you'll be able to see the imperfections on it like dust, dirt, and maybe scratches. When you again focus on your image, those glass imperfections are out of focus. You'll note that your eyes have focussed on an image of your face that is the same distance behind the glass as your eyes are in front of the glass. Sometimes it is fun to hold a meter stick perpendicular to the glass with your face at varying distances from the glass and seeing how far your face image shows on the back of the reflective surface.

Historically, Isaac Newton and Christiaan van Huygens argued about whether light behaved as a particle or as a wave. They agreed that both theories worked well to explain reflection from a plane mirror.

More Info: en.wikipedia.org