Delta is an energy conserving material. What this means is that it has realistic properties like when specularity is 100% the diffuse in turn will drop to 0%. Faster than Dielectric while offering less control, it is an excellent node for background glass or metal objects.
The surface color, often supplied by texture maps.
Specularity is the correct term for reflection. Normally all surfaces have some element of incident reflection and this setting is usually accompanied by a Roughness setting (in the Standard material this accompanying setting is called Glossiness and works in the opposite direction - 100 % Glossiness is the same as 0 % Roughness).
Defines how transparent a surface is, often paired with
Refraction Index (Scalar)
This value is for the Index of Refraction of your material. Often used - 1.33 for water, 1.5 for glass, 2.4 for diamond.
Bump Height (Scalar)
Specifies the bump height or “amplitude” of the Bump directional vectors.