Reflection is the bouncing of light off of a surface at the same angle at which it hits the surface. Refraction is the bending of light as it passes through a medium with a different density.
A lens works by bending light as it passes through it, either converging or diverging the light rays. This bending is due to the varying density of the lens material.
The law of reflection states that the angle of incidence (the angle at which light hits a surface) is equal to the angle of reflection (the angle at which light bounces off the surface).
The angle of incidence and the angle of refraction are related by Snell's law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the speed of light in the first medium to the speed of light in the second medium.
A convex lens is thicker in the middle and thinner at the edges, and it converges light rays to a focal point. A concave lens is thinner in the middle and thicker at the edges, and it diverges light rays away from a focal point.