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In the United States, the legal definition of blindness is visual acuity of not greater than 20/200 in the better eye with correction or a field not subtending an angle greater than 20 degrees.
So what does that mean?
Most people are aware that a person with normal, clear vision sees 20/20. What this means is that the person can see at 20 feet what a normally sighted person can see at 20 feet. Lots of people don't see this well, but their vision can usually be corrected with eyeglasses or contact lenses (and some with laser surgery) to 20/20. That's what the "correction" clause in the definition of legal blindness means. Even if your vision is 20/200 without your glasses, if it is better with glasses, you are not legally blind.
So what does 20/200 mean? A person with 20/200 vision has to be 20 feet away or closer to something that a person with normal (20/20) vision can see from 200 feet away. This is the equivelent to not being able to see the big "E" on the top of a Snellen eye chart. So, while a person with 20/200 (or worse) vision can see some things, like big shapes, colors, movement, they can't see it in any great detail unless they are very close to it.
The second part of the definition is visual field. Remember, you only need to qualify under one of the parts, although some people qualify under both. A person with normal vision has clear vision in the center, with gradually fuzzier vision that is mostly only good for catching movements and bright colors that shift your eyes focus to whatever it is that got your attention. This fuzzy outer vision is called peripheral vision. Some people, especially people with retinal degeneration diseases like Retinitis Pigmentosa, gradually lose this outer vision as their retina dies. They can still have clear central vision, but because it is so narrow it is restricted to a very small area, like looking through a toilet paper tube. This is called "tunnel vision" and a person is considered legally blind if this tunnel is narrower than 20 degrees.
Another way to have a restricted visual field is through central vision loss, like in macular degeneration. In macular degeneration, vision is lost from the inside out. The person still has peripheral vision, but as stated before, peripheral vision is not very clear. I am not sure how much of central vision needs to be obstructed before a person is considered legally blind, but I would assume it would be very little.
Another way to have a restricted visual field is to have permanent (from retinal degeneration, as in diabetes) or floating (as in people affected by toxoplasmosis in utero) blind spots that obscure vision. These block out spots in both central and peripheral vision.
Most legally blind people are not totally blind, as is commonly thought. Most blind people do have some degree of useful vision.