Before we had the Principled node we used to use the Glossy mixed with Diffuse node, but now the Principle node takes care of that in one node. I like to think about it this way, Specular is how shiny the object is, glossy (or roughness for the Principled) determines how that shininess behaves. Glossy to 1 means super dull or rough reflections, as you go to to 0 then it is less ans less rough, so the reflections become super crystal clear, like a mirror.
So the glossy node is the roughness in the Principled and the specular was something added to the Principled, I think to handle what we used to do with the Fresnel. But try not to compare Specular to anything that used to be in Blender. Specular is that overall behavior of the material to reflect, glossy or roughness is how that reflectivity behaves, either dull or mirror like and anything in between.
I think I don't understand the basic difference between specularity and glossiness (if there is any). Is "Specular" actually the Glossy Shader part within Principled BSDF?
No, there is a difference! Like Omar alluded to, the specularity controls how much light gets reflected in total, while the glossiness controls how sharp or soft that reflection is. A diffuse material can still be fully reflective - it's just that the reflections aren't very sharp. Turn the Glossy shader's roughness all the way up to 1 to see what I mean. It's not reflecting any less light, it's just reflecting it differently. Martin is also helpful in pointing out that Specularity will be renamed to IOR Level in the next version of Blender.
Why doesn't mixing a glossy and diffuse node give the same result as the principled shader?
The principled shader does use the diffuse and glossy shaders under the hood, but it layers them together using Fresnel in a way that's more physically accurate than just a straight mix.
I also realized that there are "Specular Workflow" and "Metallic Workflow"
This is a totally separate concept - which I agree is confusing because they use a lot of the same terms! The specular workflow actually has to do with how textures are created and how the shader is set up to interpret them. I wouldn't worry about it at this point because Blender only really supports the metallic workflow and it's only something you need to think about when you start using PBR textures from online or creating them in an app like Substance.
Hope that helps!
