Blend Modes:
The Normal blend mode simply draws the contents of the layer on top of its background. This is a good default blend mode.
The Multiply blend mode multiplies the values of the layer's colors with the background colors. The results are at least as dark as the layer and background.
The Screen blend mode multiplies the inverse of the values of the layer's colors with the inverse of the background colors. The results are at least as light as the layer and background.
The Overlay blend mode uses the multiply or screen blend mode depending on the background's color. The result reflects the lightness or darkness of the background.
The Darken blend mode chooses whichever of the layer's color and the background's color is darker.
The Lighten blend mode chooses whichever of the layer's color and the background's color is lighter.
The Color Dodge blend mode makes the background's colors lighter based on the layer's colors.
The Color Burn blend mode makes the background's colors darker based on the layer's colors.
The Soft Light blend mode lightens the background color (similarly to Color Dodge) if the layer's color is lighter than 50% gray, or darkens the background (similarly to Color Burn) if the layer's color is darker than 50% gray. If the layer's color is 50% gray, the background is not changed. This is useful for adding highlights to your image.
The Hard Light blend mode lightens the background color (similarly to Screen) if the layer's color is lighter than 50% gray, or darkens the background (similarly to Multiply) if the layer's color is darker than 50% gray. If the layer's color is 50% gray, the background is not changed. This is useful for adding highlights to your image.
The Difference blend mode subtracts the layer's color from the background's color if the background's color's brightness is greater than the layer's color's brightness, or subtracts the background's color from the layer's color otherwise. Black in the layer shows the background and white in the layer shows the inverse of the background.
The Exclusion blend mode is similar to the Difference blend mode but with lower contrast.
The Hue blend mode uses the luminance and saturation of the background's color and the hue of the layer's color.
The Saturation blend mode uses the luminance and hue of the background's color and the saturation of the layer's color.
The Color blend mode uses the luminance of the background's color and the hue and saturation of the layer's color. This is useful for tinting a grayscale image with a color.
The Luminosity blend mode uses the hue and saturation of the background's color and the luminance of the layer's color. The result is the inverse of the result of using the Color blend mode.
The Copy blend mode replaces the background with the layer.
The Source In blend mode draws the layer but only with the opacity of the background.
The Source Out blend mode draws the layer but only with the inverse opacity of the background.
The Source Atop blend mode draws the layer limited by the background's opacity on the background limited by the inverse of the layer's opacity.
The Destination Over blend mode draws the layer behind the background.
The Destination In blend mode draws the background but only with the opacity of the layer.
The Destination Out blend mode draws the background but only with the inverse opacity of the layer.
The Destination Atop blend mode draws the layer limited by the inverse background's opacity on the background limited by the layer's opacity.
The XOR blend mode draws the layer limited by the background's opacity on the background limited by the layer's opacity.
The Plus Darker blend mode draws whichever of the background's color and layer's color is darker.
The Plus Lighter blend mode draws whichever of the background's color and layer's color is lighter.