Mordor, meaning "dark land," is a land far in the east, and is the dwelling of Sauron, the Dark Lord. It was the smallest of the regions of Middle-earth, though it is almost totally consumed by evil. It is surrounded on three out of four sides by mountains. There is little to this land, but here is all the known information:
Morannon, the Black Gate, was the primary entry way into Mordor. It was only usable, however, by Sauron's own Dark Forces. Frodo, Sam, and Gollum waited in vain by this gate, hoping that it would open, but it never did. It finally opened near the end of the War of the Ring, when Aragorn and the forces of Gondor arrived. The gate was believed to have been destroyed at the time of the destruction of the Ring.
Minas Morgul is the city of the Wraiths. It was originally called Minas Ithil, before it was overcome by the Nazgûl in 2002 of the Third Age. The only change made to Minas Ithil to transform it into Minas Morgul was that the original beauty of Minas Ithil was replaced with fear and terror. The walls had a dim eery light, as well.
Cirith Ungol was the secondary entrance into the realm of Mordor. It was a great, high mountain pass, through many caves, with long stairways, that lead up to the land of Mordor. This was the entry way used by Frodo, Sam, and Gollum. It may have been destroyed at the time of the end of the Ring.
Barad-Dûr, the Dark Tower, was Sauron's lair. He did not depart from this place from Bilbo's Quest forward. This tower was destroyed when the Ring was destroyed, and thus Sauron was killed.
Mount Doom and Sammauth Naur:
Mount Doom, or Orodruin, was the place where the One Ring of Power was created by Sauron. And there, in the Cracks of Doom, the Ring was destroyed when Gollum toppled into the Fires while holding Frodo's bitten off finger with the Ring still on it. Mount Doom flooded itself in lava and destroyed itself moments after the destruction of the Ring. Sammath Naur, or "Chambers of Fire," are the rooms and chambers containing the Crack of Doom.
The Plateau of Gorgoroth:
The Plateau of Gorgoroth was no more than a flat stretch of elevated land on which Barad-Dûr and Mount Doom sat. The ground was ashen and broken, and thus was very difficult to walk upon.
Nurn was a flat land with a dark river upon it. There was nothing else there.
The Sea of Nurnen:
The Sea of Nurnen was an inland sea. It had, however, been defiled by Sauron's evil, and thus was undrinkable. Its waters were as black as the skies of Mordor. It is believed to have been dried by the lava of Mount Doom at the destruction of the Ring.
Ered Lithui, or the Ash Mountains, formed the northern border of Mordor. These were dark mountains, and they were avoided by men, elves, and dwarves.
Ephel Duath, the Mountains of Shadow, formed the western and southern borders of Mordor. These mountains were very similar to the Ered Lithui. In fact, they were almost identical.