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.
