Blend modes interact layer with the layer below to modify the effect. There are 27 Blend Modes, but most are not especially valuable for photographers. There are four more articles on blend modes that describe in greater detail the blend modes most useful to photographers. This article is a very simple overview of all the blend modes available in Photoshop. There are no samples in this article.
Normal - no change at 100%. At reduced opacity the layer underneath becomes visible relative to the density of the top layer. No math is involved in the decisions except the layer opacity.
Dissolve - no change at 100%. No pixel blending math. At reduced opacity underlying pixels are revealed through a dither pattern which introduces noise. Not of much value to photographers.
Darken Modes: White is invisible (produces no change).
Darken - result colors will be darker. White will be invisible. Compares the layers and returns the darker of the two based on the values of individual pixels. No blending is done.
Multiply - multiplies the lower layer luminosity value and blend color of the top layer resulting in darker tones. Similar to a two slide sandwich. White produces no change. Blacks remain black. Useful in many ways for darkening an image or part of one.
Color Burn * - Darker result than multiple adding contrast which results in more saturated midtones but reduced highlights. Similar to the burn tool results but can be used in a more controlled manner.
Linear Burn * - Darker than Multiply but less saturated than Color Burn - most contrast is added in darker values.
Darker Color - Compares individual color channels and the result is the darker color by channel. Differences are minor compared to Darken mode because the math is different. Darker Color uses the composite color of a pixel rather than the individual color channels
Lighten Modes: Black is invisible (produces no change).
Lighten - result colors will be lighter. Black will be invisible. Compares the layer with below and returns the lighter value.
Screen - opposite of Multiply. Always a lighter value. Useful in brightening an image or a part of one.
Color Dodge * - generally results in blown high values, and increased saturation in midtones. Similar to the dodge tool effect but with greater control through masking.
Linear Dodge * - a stronger result than Screen. Can be used to open shadows with low opacity.
Lighter Color - compares base and blend and returns the brighter of the two using the composite values rather than individual channel values. The opposite of Darker Color.
Contrast Modes: darkens if darker than 50% gray - Lightens if lighter than 50% gray. %0% gray is ignored except by Hard Mix. The effect in an increase in contrast and the results depend on the math used by each mode.
Overlay - a combination of Multiply and Screen with reduced effect of each. The results is a strong increase in overall contrast. The base layer colors are used to create the effect. Other contrast modes use the blend layer values instead.
Soft Light - similar to overlay with less contrast. A very useful mode.
Hard Light - intense increase in contrast resulting from using the blend layer values for calculation.
Vivid Light * - extreme version of Overlay - rarely useful at full opacity.
Linear Light * - uses Linear Dodge on lighter values and Linear Bur on darker pixels resulting in an even more intense contrast boost. Useful in Frequency Separation technique with a layer primarily composed of middle gray values such as those produced by the high pass filter.
Pin Light - tends to remove midtone values resulting in unusual patches of color depending on the values in the top layer. Little value to photographers.
Hard Mix - result can only be black, white or one of the six basic channel colors. Of little value.
Difference * - white inverts color value. Black does nothing. Putting the top layer of two identical images (content) is useful in checking for alignment.
Exclusion - similar to Difference except that 50% gray produces 50% gray. Lighter values tend to invert the colors. Darker values tend to gray down blacks.
Subtract - subtracts the top layer pixel value from the base layer. Black has no effect. Lighter tones darker the image overall by subtracting the brightness from the underlying pixels. White simply turns the image black.
Divide - strangely enough it is the opposite of Subtract with the effect being a lightening of the image. Black turns the image pure white.
Hue - blends the top layer color into the image without modifying the base luminosity. Maintains the saturation level of the base layer at 100% opacity.
Saturation - basically retains the color (hue) and luminosity of the base layer while introducing the saturation values of the top layer. Therefore neutral grays of any luminosity convert the image to black and white as saturation is removed.
Color - maintains the base layer luminosity while introducing the blend color. Useful in modifying the color of a selected area while retaining the detail and luminosity, especially at lowered opacity.
Luminosity - maintains the base layer colors while modifying the luminosity values. Therefore the color of the top layer is ignored while the brightness is modified. Vignettes and similar modifications to an image in Luminosity mode will avoid color shifts.
Pass Through - this mode is applied automatically when you create a group from layers. That retains the expected behavior of the individual layers within the group relative to the underlying layers. Setting a group to Normal mode only allows the contained layers to affect the group and not the underlying image layers.
* These modes produce different results when the Fill opacity is reduced than when overall opacity is reduced.