SINGLE VISION is a lens with one point of focus. Single vision lenses are prescribed for focusing at one distance.
BI-FOCALS are lenses with two focal points. Commonly prescribed for distance vision and reading vision. They have a segment for which plus power is added for shortening the focal length. With this additional segment, a very distinguished line is present.
TRI-FOCALS are lenses with three focal points. Commonly used for distance, intermediate, and near vision. Like bi-focals, they also have a segment which plus power is added for shortening the focal length. However the middle or upper portion of the segment has half the power of the lower portion, allowing vision for intermediate use. This gives two lines to have to view through.
PROGRESSIVES are lenses with an infinite number of focal points. This allows vision from distance to near and everything in between. The advantage of progressives is that there is no line separating distance vision from near vision. Just a gradual increase of power to allow good vision at any given focal length.
OCCUPATIONAL LENSES can be prescribed for a hobby, computer use, or anything that you are doing that requires the need for vision at a certain distance for a long period of time. Different lens styles are available to maximize comfort for your particular needs.
There's more to lenses than meets the eye. 
