The Realms of Oblivion are Dimensions, different versions of Nirn itself. They're not planes separate from Nirn, but they ARE, Nirn. But, my personal theory regarding the Realms of Oblivion. What the Canon explains is these realms are planes of existence, both there yet not there, all forms of fancy mind boggling stuff. You can however get there by means of Portal and other forms of "Rifts". The realms of Oblivion are separate from the Mundus, you can't get to one by conventional means, ie. So, to recap, and to describe it in our real world terms, Nirn is a habitable planet with two moons orbiting it's parent star Magnus. The night sky has stars in all directions, and the day sky has the sun, Magnus, which Nirn appears to be orbiting. Nirn (perceived from the Mortal eyes on the ground) has two moons that appear to orbit Nirn, Masser and Secunda. It's nearly identical to our Earth, land, oceans, a sky, breathable air, weather, climates etc. Nirn is a life sustaining world, similar to a planet. I'm gonna go out on a limb now, a brittle, precarious limb, and try and start a discussion, (which I kinda already did once already which ended poorly.) about the Realities behind Nirn, the Mundus, Oblivion, Aetherius, and the greater cosmos this Universe inhabits.