What do they all do and when should we use them?
The Normal blend mode is default. The two layers do not interact in any way at all.
Dissolve makes the lower layer take on the colours of the top layer, depending on the Opacity of the top layer, which is 50%, here.
Darken compares each pixel between the two layers and displays the darker of the two.
Multiply darkens the lower layer based on the darkness of the upper layer.
Colour Burn burns the colour of the two layers, to create a darker image.
Linear Burn is similar to Multiply, only it applies a more intense finish.
Lighten compares each pixel between the two layers and displays the lighter of the two.
Screen brightens a picture by lightening a layer based on the upper layer. It’s the opposite of the Multiply.
Dodges the lower layer with the upper, to give a lighter image.
The same as Screen, just more intense.
Overlay multiplies light colours and screens dark colours.
Soft Light multiplies dark tones and screens light ones.
Hard light does the same as Soft Light, just with colours and not tones.
Vivid Light dodges or burns lower layers depending on whether the upper layers’ pixels are bright or darker than a basic grey.
The same as Vivid light, but works on brightness and not contrast.
Combines both Screen and Multiply depeding on whether or not the upper layer is darker or lighter than basic grey.
Inverts the colours.
Uses the darkness of the lower layer to mask the difference between two layers.
Hue changes just hue: not brightness or saturation.
Alters the saturation of a lower layer, changing it to the hue of the upper layer.
Changes hue and saturation.
Changes luminosity of a lower layer to the liminodity of an upper layer, but changes nothing else.