Map of the Nirn
Nirn is largely known by otherworldly beings as Mundus, or the mortal plane. The exact reasons and specifics of Nirn's creation are largely debated, but all agree that the deity Lorkhan was the mastermind behind its creation. How he created Nirn is also unsure, ranging from the other Aedra agreeing to willfully help Lorkhan to him tricking them into it. However, it is universally acknowledged that the Aedra gave their energy and power for it, and that the majority gave up on the project after realizing the cost, and that those who didn't became the Eight Divines, or the Earth Bones.
From top to bottom: Masser, Secunda.
It is orbited by two twin moons, Secunda and Masser
Nirn seems to be smaller than Earth, whose Moon could fit inside Nirn approximately four times. It should be noted that as Masser (Nirn's larger moon), revolves around Nirn. It can be spotted that in Oblivion Secunda (Nirn's smaller moon), does actually revolve around Masser. Thuse some opinions say that Masser is actually a planet. But since it is revolving around a planet it is a bio-revolving moon system.
The comparison of Earth and Nirn is inaccurate, for the actual size of Masser is unknown.
Geography
The notably large continent on the planet is called Tamriel, which is where all of the Elder Scrolls games takes place. With an another similarly large continent to the east of Tamriel, this continent is named Akavir. The north of Tamriel is the cold barren land called Atmora, which is where the Nords of Tamriel hailed from. To the west of Tamriel lies Yokuda, which was the former homeland of the Redguards of Tamriel.
Tamriel
Tamriel is geologically interesting as it has mountains, tundras, cold deserts, hot deserts, swampy regions, grassland, tropical rainforests and an arguably large volcano. The diversity of biomes is unique as the hot desert is bordered by cold mountains to the north.
Tamriel also holds home to a series of islands called the Summerset Isles and is where the Altmers or High Elves hailed from.
Yokuda
Not much is known about Yokuda since it was nearly obliterated in a cataclysmic event, but it seems to have a dull, rocky, hilly landscape.
Atmora
Atmora is a very cold region, and this caused the water of the Sea of Ghosts (which borders Tamriel and Atmora) to become icy. It is a barren freezing wasteland.
Akavir
Nearly to nothing is known about the geography of Akavir.