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I am trying to understand Kirchoff Plate Theory and I am unable to understand the difference between bending and twisting moment for a plate. In a beam structure, if moment is along the axis of the beam, it creates twist, and if the moment is normal to the axis of the beam, it results in the bending of the beam. However, how do we differentiate between moments when dealing with plates.
Thank you.
Bending moment creates axial compressive and tensile stresses above and below the mid-surface and shear normal to mid-surface parallel to surface loading, q. In the most basic case roughly like a plate supported at the end acting as a wide beam.
Twisting moment causes in plate shear and normal stresses, I attach a very simple diagram for that. This is a case of twisting on a differential section of a cylinder plate under torsion.
I attache the diagram here. 
