When someone has diabetes, the result is damage to the small blood vessels in the body. When these vessels become compromised the body creates new ones to try to bring nourishment to the area. These new vessels are not very strong and tend to leak fluid or blood.
The blood vessels in the eye are all small so this new vessel growth and leakage is very common. It results in many problems with vision. If the blood vessels are in the front of the eye it can lead to glaucoma. Usually they are in the retina and underlying layers. (Diabetic Retinopathy) Tiny aneurysms develop and can bleed into the center of the eye, causing very cloudy vision. Sometimes this blood absorbs and sometimes it requires surgery or laser treatment to remove it.
Sometimes this leaking of fluid occurs under the macula and causes swelling. (Diabetic Macular Edema) The functional effect of this is the same as the effect of macular degeneration. Depending on how well the diabetes is controlled and how much damage occurs to the retinal vessels the vision loss can be minor or very debilitating.