Have you ever observed birds or animals encountering a mirror? They misidentify their image as someone looking at them from the backside of the mirror. If possible, they run around the mirror to verify whether someone is there! Of course, they find nothing there and get puzzled by this behavior. An image that appears to be there but is not actually there is called a virtual image. Contrast this with an image formed by a camera lens. When the lens of the camera forms an image of an object, the image is real. That’s why we can put a photographic plate or sensor at the image location and actually record the image.
Suppose you are standing in front of a verticle mirror. The image of your face will be behind the mirror at a distance equal to the distance between the mirror and your face. This is because the light travels in a straight line and reflects back when it hits a mirror at an angle equal to the angle at which it was incident on the mirror. The light rays from the different parts of your face reflect back from the mirror and our eyes detect them. Yet, our mind doesn’t interpret the whole event this way. The mind creates a perception in which the light rays traveled from the other side of the mirror toward our eyes. Hence, the rays appear to form an image behind the mirror.
If the mirror is a plane, all the reconstructed points on the other side of the mirror are of the same size and at the same distance from the mirror as their corresponding points on your face. This is because of the fact that the rays are reflected at the same angle at which they were incident on the mirror. If the mirror is not a plane, the reconstructed points on the other side of the mirror will produce a distorted image of your face. Visit a hall of mirrors, if you have not, to witness this effect.
Sometimes we have to ignore the perception of virtual image created by the mind reconstruct the location of the original objects by looking in the mirror. This is a difficult process requiring conscious thinking. One simple example is cutting our own hair by looking into the mirror. You will notice how difficult is it to judge the location of the hair and the scissors. The problem doesn’t arise in combing the hair. We have combed our hair for such a long time that our minds have adjusted to this reconstruction in this case. Another example is driving your car backward by looking in the mirrors.
(My answer to a question at Quora)