A single vision lens is a lens that serves only one purpose. It is usually used for either reading or for seeing in the distance. The most common material that a single vision lens is made out of is CR39 (plastic), however, they are also available in glass or photochromatic or Transition Lenses (a plastic that darkens in the sun).
A bifocal lens is most commonly described as the lens with the line on it. The top section is used for the patient to see in the distance. The bottom section, which is called the seg (segment), is used for reading. There are different types of bifocals, but the most commonly used is called the 28mm D-Seg.
A progressive lens is one that increases in power gradually towards the lower portion of the lens. It is not divided into sections, hence has no visible lines on it. When looking straight ahead, you should be able to see in the distance. When you move your eyes towards the bottom of the lens, you should be able to read. In between, you should be able to look at intermediate distances clearly. This lens blends from your distant vision to your reading.