Confused with PBR implementations
DFG pops up in the family of microfacet based BRDFs. It is simply the product of three terms:


D : The microfacet distribution.
F : The fresnel coefficient.
G : The geometric attenuation between microfacets.


When someone says Cook-Torrance, they usually mean a microfacet BRDF where the distribution (D) is Beckmann, which I think is what the original Cook-Torrance paper was about.

GGX is really just a different distribution (D term). GTR is another one. The names are often used as a shortcut to mean a BRDF using that distribution.

Now all of these assume specular reflection (or refraction) on the microfacets, hence the fresnel term. If you take a similar idea but use diffuse (lambertian) microfacets, you get the Oren-Nayar BRDF. I think that one is based on a gaussian distribution of the microfacets but I'm not familiar with the details.