The document describes two types of lenses: converging lenses, which are thicker in the center and can form real and virtual images, and diverging lenses, which are thicker at the edges, creating erect and reduced images. It also introduces the concept of the principal axis, which is the line joining the centers of the two spheres that form part of the lenses. Additionally, it discusses the different image formations in relation to the object's position relative to the focal point.