Diabetic retinopathy has four stages:Mild nonproliferative retinopathy: At this earliest stage, microaneurysms occur. They are small areas of balloonlike swelling in the retina's tiny blood vessels. Moderate nonproliferative retinopathy: As the disease progresses, some blood vessels that nourish the retina are blocked. Severe nonproliferative retinopathy: Many more blood vessels are blocked, depriving several areas of the retina of their blood supply. These areas of the retina send signals to the body to grow new blood vessels for nourishment. Proliferative retinopathy: At this advanced stage, the signals sent by the retina for nourishment trigger the growth of new blood vessels. This condition is called proliferative retinopathy. These new blood vessels are abnormal and fragile. They grow along the retina and along the surface of the clear, vitreous gel that fills the inside of the eye. By themselves, these blood vessels do not cause symptoms or vision loss. However, they have thin, fragile walls. If they leak blood, severe vision loss and even blindness can result.
This answer is based on source information from National Eye Institute.