From L->R: Phong shading; Blinn-Phong; PBR with directional light; PBR with IBL
"Reflectivity" is the measure of how much the material substance reflects at angle 0 to the light vector. Energy conversation states that as we increase the reflectance property of a material, the albedo (diffuse) reflection should reduce proportionally
"Gloss" (in PBR) is a measure of the microfacet roughness of the surface, varies both width and intensity of highlight, as opposed to just width in standard Phong glossiness.
