|“||On the eastern bank to their left they saw long formless slopes stretching up and away toward the sky; brown and withered they looked, as if fire had passed over them, leaving no living blade of green: an unfriendly waste without even a broken tree or a bold stone to relieve the emptiness. They had come to the Brown Lands that lay, vast and desolate, between southern Mirkwood and the hills of Emyn Muil.||”|
–The Fellowship of The Ring, The Great River
During Sauron's first war, he adopted the 'scorched earth' policy and destroyed everything he could find whilst his dark army withdrew from the west. He came upon the gardens of the Entwives, which lay east of the Anduin, and annihilated them.
The Brown Lands are the result of that meaningless destruction, a desolate area scattered with boulders and dead oak trees. The grass here is brown (in case the name could not convey that.) It is covered in barren dirt and scrubby grass and is rather dull. It is similar to the Dead Marshes, another place ruined by Sauron, other than the fact that there is very little water (or anything else for that matter) here. The Brown Lands were once home to the gardens of the Entwives; that is, until those gardens were incinerated by Sauron.
Upon entering the Brown Lands you gain the achievement "Barren Wasteland".
There are no mobs whatsoever in the Brown Lands. Not even the Orcs can stand this place, though they do invade it if the enemies of Mordor dare to enter Sauron's lands.
The grass here is a predominantly short, dull brown and interspersed with barren patches of dirt. The oak trees here are dead. Boulders dot the landscape. No living things exist in the Brown Lands.
The Tolkien Gateway has an article on: