Photographer and blogger Robert Thomas has posted a comprehensive description of each of Adobe Photoshop's blending modes and how they affect your image. Since Photoshop CStotal of, this seems like a good article to bookmark and read and come back to regularly for reference.
Before I list thehow they work, you need to understand how the math in Photoshop works. Because the blend modes work with brightness and darkness values, i.e., luminance levels, and because luminance levels appear as values from(as seen with the Levels dialog box), you would assume that the math Photoshop performs is based on these values. However, in the background, Photoshop “standardizes” these luminance values before applying the math. When these values are standardized, white (255) becomes 1, black (0) remains at 0, and 50% gray becomes 0.5. All of the blend mode math takes place in the small range between1. It’s important that you understand this concept of standardization, so you can understand the following mathematical equations.