A lens is a portion of a transparent refracting medium bounded by two surfaces. Depending upon the nature of surfaces lens may be of following types.
Convex lens has its two surfaces bulging outward. It makes the parallel rays of light to converge to a point. Hence, it is called converging lens. The point of convergence is called focus.
A concave lens has its two surfaces caving inward. It makes parallel rays of light to spread from a point. Hence, it called diverging lens. The point where from light rays appear to diverge is called focus.