Mordor was the region east of the Anduin where Sauron chose to establish his stronghold beginning in the year SA 1000.
It was bounded on three sides by mountains; impassable except for in two places. The Ash Mountains (Ered Lithui) marked the northern border, and the Mountains of Shadow (Ephel Dúath) acted as the border to the west and south.
The main access to Mordor lay in the northwest corner at the point where the two mountain ranges meet. From the Dagorlad, entry was through the Morannon and Cirith Gorgor into the bowl of Udun and then through the Isenmouthe to the interior of Mordor. In the west the pass known as Cirith Ungol also allowed access to Mordor, through the Mountains of Shadow.
The interior of Mordor was divided into two regions. In the nothwest of Mordor lay the Plateau of Gorgoroth; a dry, barren, fissured, volcanic plateau. Mount Doom and Sauron's fortress of Barad-dur were in this region and it was on this plateau that Sauron located his forges and mills for the construction of the armaments for his troops.
To the south and east of the plateau of Gorgorath lay the Ash Plain (Lithlad), a fertile region drained by rivers flowing into the Sea of Nurnen. The two regions were separated by spurs jutting to the southwest from the Ash Mountains and to the East from the Mountains of Shadow. Food for Sauron's troops was grown by slaves on farmland around the Sea of Nurnen. After the destruction of the Ring, Elessar freed these slaves and gave them this region as their own.