Diabetics are at a higher risk for developing problems with their eye and vision. To decrease the risk, diabetics are encouraged to make regular eye appointments and more importantly, to take great care in managing their sugar levels!
Diabetics can have too much sugar (glucose) in their blood and either not enough or dysfunctional insulin. The lens tissue in the eye doesn't need insulin to uptake glucose (note: most cells require insulin in order to uptake glucose). When their is too much sugar in the blood, the lens keeps taking up glucose, even though it doesn't need the excess glucose. Glucose never travels alone, so every molecule of glucose that is taken up into the lens, a molecule of water goes with it. This leads to an excess of water in the lens, which causes the lens to swell. The lens actually doesn't have room to expand and swell, thus the lens cells get scarred and damaged due to the excess water and associated pressure pressing on the lens. The scarring of the lens cells can cause blurred vision. Also, an accumulation of scarred lens cells can develop into a cataract. A cataract is a cloudy spot on the eye and it can also decrease vision.
Other eye concerns (other than scars and cataracts) that may develop as a complication of diabetes include Glaucoma (increased eye pressure, can damage the optic nerve, can lead to blindness if not treated) and Retinopathy (inflammation of the retina, which is the neural layer of the eye. The retina takes the image we see and sends it to the brain).
Rather than make this a ridiculously long and science-y blog post, I'll summarize what happens and say that in diabetes, all of the excess sugar traveling in the vessels gets subject to metabolism and becomes modified (glycosylated). These odd-shaped glycosylated end-products can become stuck in vessels. Since there are many vessels in the eye (the eye is well vasculated), these vessels can become clogged by the end-products, blocking blood flow to the eye. A blockage in these vessels can eventually disrupt blood flow to the eye, starving parts of the eye and causing the development of more vessels (neovasculization), and these two can trigger the development of these eye diseases (Retinopathy and Glaucoma).
Alas, the development of these diabetes-associated eye problems is really quite complicated, but I tried to spare you the details and just cut-to-the-chase. Hopefully I didn't lose you along the way and that you still got something out of this post. :S