Convex lenses have a thicker centre than their edges, and concave lenses have a thicker centre than their margins. A convex lens focuses a light beam travelling through it on a spot on the other side of the lens. The focus point is the name given to this place.
A convex lens functions similarly to a concave mirror. Both have positive focal lengths, converge parallel rays to a focus point, and produce pictures with identical features. A concave lens functions similarly to a convex mirror.