The lens types differenciate the strengths and focus amount types of a lenses. A lens can have none, up to three different areas of focus. These areas are plano (non-prescription), single vision, double vision (or bifocal), multifocal vision (or trifocal / progressive).
Common single focal lens used for distance, near, reading, computer, intermediate and astigmatism. Single vision lenses are your day-to-day prescription lenses for farsightedness or nearsightedness with a single area of focus.
As opposed to traditional multifocal lens with the lines, progressives are discrete and multi-functional. Commonly referred as no-line bifocal or multifocal lenses. Multifocal progressive lenses function with three areas of focus - distance, intermediate and near all blended in one lens. These three focals blend into each other from top to bottom, without any obvious lines. The dotted portion of the lens edge is referred to as the blending area, and not intended as a viewable area to look through.
Traditional lined bifocal lenses with with two focal areas - Distance and near. The majority of the lens area is distance with a half-circle reading area near the bottom. The reading portion of the bifocal area comes in different shapes and sizes. One of the most common (shown) are the flattop 28 style. To see different types of bifocal styles click here.
Lined trifocal lenses are made similar to lined bifocals, but also has a smaller third area of lens strength for intermediate reading area above the near area. Trifocal lenses function similar to it's discrete option - progressives by offering a multifocal solution with three focal areas.
Plano lenses are basic single non-prescription lenses most commonly used for cosmetic or safety reasons.