According to the National Dissemination Center for Children with Disabilities (NICHCY)  the blanket term "visual impairment" includes low vision, partially sighted, legal blindness and total blindness. These terms are discussed below.
Low vision is used to describe a loss of visual acuity while retaining some vision. It applies to individuals with sight who are unable to read a newspaper at a normal distance of viewing, even with the aid of glasses or contact lenses. People with low vision often need adaptations in lighting and/or enlarged print to read something. There are two specific types of low vision:
- Myopic- Unable to see distant objects clearly, commonly referred to as "near-sighted."
- Hyperopic- Unable to see close objects clearly, commonly referred to as "far-sighted."
Partially Sighted is usually used in educational contexts to describe a visual impairment that requires special education services. The partially sighted student meets the challenge of disability in much the same way as a totally blind student. Accommodations include the use of readers, audio taped texts, and raised-line drawings. The partially sighted student may be able to use large print books and a Closed-Circuit TV (CCTV) or other magnifying device. Some partially sighted students can take notes in class by printing very large with a felt tip pen or marker; others will tape record lectures for later use.
Legally Blind refers to people that have less than 20/200 vision in the better eye or a limited field of vision that is 20 degrees or less at its widest point. People who are legally blind may have some useful vision.
Totally Blind individuals need Braille, raised-line drawings, audio recordings, and/or other non-visual media as an accommodation for accessing the content of visually presented materials.
The descriptions above were compiled using the following resources.
The NICHCY publication Visual Impairments. 
The American Foundation for the Blind (AFB)  website.
The The National Federation of the Blind (NFB)  website.
Braille Plus, Inc.'s glossary  of definitions and terms related to braille, visual impairments and blindness.
-  National Dissemination Center for Children with Disabilities (NICHCY)
-  Visual Impairments.
-  American Foundation for the Blind (AFB)
-  The National Federation of the Blind (NFB)
-  glossary