Keratoconus is a condition where the clear front surface of the eye, called the cornea, starts to become thin and bulges forward. There isn't a definitive cause of keratoconus but it is thought to be linked to genetics and/or vigorous eye rubbing, generally in childhood.
Because of the irregularity of this new cornea, clear vision is hard to obtain with glasses. Symptoms of keratoconus can include blurred vision, ghosting of images, and frequent squinting.
Rigid contact lenses are the best way to obtain clear vision with keratoconus. The reason for this is because tears fill the area between the abnormal cornea and the spherical contact lens, essentially creating a "new" corneal surface where light rays can travel to the retina as straight as possible, creating a clear image.