Learning to harness the fires of the mountain Orodruin, Sauron forged the Ruling Ring during the middle years of the Second Age; this granted him the strength and power he needed to send forth his hosts and ravage the lands of Middle-earth. But at the end of the Second Age, he was cast from his Throne, and his spirit sent into exile. For many years afterward, Mordor returned to a state of desolation. (Read more in the entry on Sauron)
In his absence, Gondor built fortresses (Durthang, the Towers of the Teeth, and the Tower of Citith Ungol) in the Third Age to prevent any evil things from leaving or entering the Black Land. But after the Great Plague of 1636, these fortresses were abandoned.
Meanwhile, Sauron had re-awoken and began to plot his return to Mordor. While he did not openly reclaim his land, he instead dwelled in secret in Dol Guldur, sending his chief servants (the Ringwraiths) to prepare Mordor for his return. For the next thousand years, Sauron grew in power, and after a nearly three-thousand year absence, finally returned to the Black Land.
During the War of the Ring, the One Ring was unmade in the fires of Orodruin, many of Sauron’s works were destroyed, and the land was ravaged by earthquakes. After the Downfall of Sauron, Mordor fell into final ruin: deserted once more, the land was never made habitable again.
* Other points of interest in Mordor include Lithlad, Núrn, the Sea of Nurnen, Cirith Gorgor, the Isenmouthe, the Morgul Pass, and the Morgai.