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Diffuse - contains the colour information of the model. Mask - might be used for several purposes, but usually it selectively masks various parts of the model to obtain certain effects. It usually works in combination with other textures. Normal - adds details to the model without increasing its geometric complexity.
As to your second question - i'm not sure about anyone, but majority of graphic artists can author textures without any problems. That's what many of us do for living.
