PBR

= PBR = Physically-Based Rendering


 * Albedo is pure colour, no lighting information baked in like we used to do with diffuse maps.
 * Roughness is calculated in a specifically measured way and requires a 0-1 input only. Gloss can interpolate that input to a greater range where Roughness "shouldn't".
 * Reflectance is again, a measured value for a material. there are plenty of sources online which you can get the measured reflective colours for various materials. typically speaking, most non-metals fall into a white 0.04 range, while metals (having no albedo and being pure reflection) have much higher, coloured reflectance. Specular is something that, along with gloss, artists have to fudge to get a pleasing look.
 * Overall, PBR is more about measured results rather than mixing in 3 different maps in different proportions to achieve a look.

Tutorials

 * Physically Based Texturing by Andrew 'd1ver' Maximov
 * Physically Based Rendering for Artists by Andrew 'd1ver' Maximov
 * Rendering Remember Me on fxguide.com
 * Tileable Dirt & Pebbles Texture - Polycount Forum - a process for developing PBR textures
 * UDK Physically Based Lighting! - Polycount Forum
 * Physically based rendering in games - Polycount Forum
 * Ready at Dawn's Material system - Polycount Forum
 * TGA Physically Based Lighting CGFX Shader for Maya Viewport - Polycount Forum