The diffuse color sets how bright the surface is when lit directly with a white light source (intensity at 100%). It defines which percentage for each component of the RGB spectrum which does not get absorbed when light scatters beneath the surface. Metal's normally have a black or very dark diffuse color, however, rusty metal's need some diffuse color. A diffuse map is usually required.
The diffuse weight.
The diffuse component follows an Oren-Nayar reflection model with surface roughness. A value of 0.0 is comparable to a Lambert reflection. Higher values will result in a rougher surface look more suitable for materials like concrete, plaster or sand.
Backlight provides the effect of a translucent object being lit from behind (the shading point is 'lit' by the specified fraction of the light hitting the reverse of the object at that point). It is recommended that this only be used with thin objects (single sided geometry) as objects with thickness may render incorrectly.
In certain situations backlighting may work fine with thickness (ensure that the diffuse ray depth level is above 1).
Specify whether Fresnel affects the diffuse component.
The amount of diffuse light received from direct sources only.
The amount of diffuse light received from indirect sources only.
When rendering diffuse surfaces it is very important that the normals of the geometry face in the right direction. In the example below you can see the difference between normals that are facing inwards in the wrong direction (left side) versus those that are facing correctly in the outwards direction (right side).