Two of the most common vision problems are Near-Sightedness (Myopia) and Far Sightedness (Hyperopia).
A person who is near-sighted is able to see near objects clearly but unable to see far objects clearly.
Persons with near-sightedness have long eyeballs and hence when the the light rays come together at the focus it falls in front of the retina. This problem may be corrected with a concave lens.
A person who is far-sighted is able to see distant objects clearly but not able to see close objects clearly.
In persons with far-sightedness the eyeballs are short and when the light rays come together at the focus they fall behind the retina.
This problem may be corrected with a convex lens.