What's the difference between Glossy and Glass shaders in Eevee? Also What's the difference between Glossy and Glass shaders and Roughness parameter in Princepled BSDF shader?
From left to right, is Glossy, Glass, and "Shiny". All 3 have roughness at 0.
Roughness in Principled BSDF determines how "shiny" or "polished" the surface of somthing looks. It bears no effect on the underlying colors.
Glossy is the equivalent of metallic - the color determines the hue of the specular reflection (how much of its own color is reflected back with the light). Default (white) Glossy with roughness 0 is the equivalent of Pure Metal (White basecolor, Roughness = 0, Metallic = 1) on Principled BSDF.
Glass essentially a Glossy shader, but with a transparent BaseColor, and some refraction added which also gives transparent/refracted reflections. It also includes a Fresnel to alter the refractions near the "edge" of objects. It is one of the best ways of replicating glass in Cycles, but has its limitations in EEVEE.
The most obvious difference between Glass and Glossy is that glass lets light through so you can see things on the other side, while glossy merely reflects light.
The Glass and Glossy shaders are more like building-blocks that are used to assemble more sophisticated shaders. No material is 100% glossy for example, and the apparent gloss of a material actually varies with the viewing angle, as per the fresnel effect.
The Principled shader is pretty much functional right out of the gate. It creates, with in certain limitations, materials that behave realistically in terms of their interactions with light. This includes important differences between how dielectric materials behave vs. metallic materials.
As you can see above, the Glossy shader behaves a lot more like a metallic material. One of the big differences between dielectric and metallic materials is that on dielectric materials, the specular highlights remain white regardless of the material's color, where as metal materials have specular highlights that are tinted by the metal color.