PBR or physically based rendering is the process of rending using a more accurate flow of light. This results in images that look more photorealistic compared to just having diffuse and specular lighting. To implement PBR, we update our lighting equations to use BRDF(Bidirectional reflectance distribution function). The BRDF for diffuse light is almost the same as the one we are using previously, so we need not update that. But the BRDF for specular light is updated to use Fresnel equation. The total BRDF is given as
f(l,v) = [D(h) * F(l,h) * G(l,v,h)] / [4 * |n⋅v| * |n⋅l|]
The Blinn-Phong (D(h)) and Fresnel (F(l,h)) consider only the active microfacet normals instead of the normal for the whole surface, so the final output will be more natural.
Below you can see the change in specular details when the material smoothness is changed.
Gloss Map / Smoothness Map / !(Roughness Map):
Instead of changing the smoothness on the whole surface, we can change it on parts of the surface using a gloss map. The gloss map a Texture, but is encoded in greyscale using BC4_UNORM. We sample the gloss map the same way as we sample a texture and use the resulting value as the amount of smoothness to be used for a fragment. Below you can see a gloss map in action.
Update: Updating the gloss map, since the previous one was hard to visualize