Núrn was a semi-arid place due to its close proximity to Gorgoroth and consisted mostly of short grass landed plains, fertile enough for farming to support the armies of Sauron, as opposed to the arid plateau of Gorgoroth, which is north-west of Núrn.
Núrn was partially inhabited by Westron-speaking men that were enslaved by the Dark Lord Sauron. It was fertile enough to feed Sauron's numberless hordes during the War of the Ring. Its soil had been enriched by ash from Orodruin's eruptions, and its grasslands were fed by water from the Sea of Núrnen.
Many of Mordor's beasts resided in Núrn, among them Graugs, Trolls, Wargs, Caragors, Ghûls, Hell-hawks, Fellbeasts and a host of other nameless horrors. Before the War of the Ring, the land was the domain of tribesman under the rule of Lady Marwen, Queen of the Shore.
After the overthrow of Sauron, the peoples of this land presumably gained their freedom and might have been allowed to govern it for themselves.