Diffuse surface is the one that doesn’t have particular direction where it reflects light. It basically reflects it everywhere, so looks matte. Think about raw paper, for instance. Diffuse layers are good as the base ones, defining the material’s base color. When you create a layer in Owlet, it is diffuse by default:
You can see all the diffuse layer properties at the right side of the panel.
There parameters are common for all types of layers in Owlet:
You can read more about layers masks here.
The Color parameter is the main parameter of the material, as it defines its appearance. You can drag an image there or simply set the color, or do both if you want to tint the image.
Here are three balls, the left has stone texture, the middle one has orange tint and the right one has both:
In most cases color is the only thing you need to configure in diffuse layer block.
These parameters are used to make thin translucent diffuse objects, like paper or leaves. They let you configure a sort of subsurface scattering on the layer level. You may want to load a special texture for opacity to control how much the light is blurred by the material.
See here for more details.
This is simple normal-based bump that you can use to make flat surfaces more interesting. You can load both height- and normal-maps (switched in the texture editor window) and configure the level of the effect by adjusting the numeric field.
Here are the two planes with the same bump texture, but different bump levels:
Bump level is measured in centimeters.