I don't really know. Well, I know what it is to me, so that is what I will try to explain. Google it. It is complicated. A disease of the retina. No treatment and no cure.
What I know is that by the time I was diagnosed with it the retina specialist thinks I am under 5-10 years from being legally blind. How did she make this decision? Well, she guessed. Because the disease is different in everyone. It progresses at different rates, and it progresses to different levels of severity. Very few, become totally blind, about 4-5%. Most are legally blind by 40.
Basically your peripheral vision closes in and you have "tunnel vision." Think looking through a straw. Also night blindness is one of the first indicators of the disease. In the late stages of the disease, it can effect your central vision.
Just to clear this up. Legally blind means, legally blind even after correction and surgery. So you and your brother are not legally blind, unless you are legally blind with your glasses and after your cataract surgery. Man, that sounded grumpy. Maybe it's cocktail time!