What is the difference between a lens and a mirror?

A mirror is a reflective surface. Light passes by the glass and hits the silver backing, reflecting off of it. (The angle of reflection equals the angle of incidence.)

A lens, on the other hand, does not reflect light; it refracts it. After entering the glass, light refracts in a different way depending on the shape of the lens and also makes a focal point where the refracting light comes to a point. This varies for different lenses -- convex or concave -- which is why there are dissimilar lenses for different types of eyeglasses.