In developed countries most visual impairment is caused by age-related conditions, such as macular degeneration or cataracts. This means that most visually impaired people are over retirement age, and have lost their sight gradually. In the developing world, most blindness is caused by easily preventable malnutrition and disease. Under-nourished children are as likely to become blind as older people. Basically, it's a question of degree.
Blindness is a substantial and permanent lack of sight.
Partial sight is a less severe loss of vision. On this page, the term 'visual impairment' is used to describe both blindness and partial sight. I can only see colours, so if I go into a dark room it feels strange because I'm used to seeing colours all around me. In my classroom I can see light colours like the window, and if a light is on I can see that as well. People sometimes fuss about me being blind, but it really doesn't matter at all to me.