The Easterlings were the inhabitants of Middle-Earth's far eastern lands and dedicated servants of Sauron, who aided him in his war on the rest of Middle-Earth, in particular Gondor, the Easterling's sworn enemies.
The Easterlings were descended from men who had refused to make contact with the Elves and instead isolated themselves in the lands of the Far East. The Easterlings themselves were cruel and dedicated servants of Sauron, who benefitted a great deal from serving him loyally. They also despised Gondor and thus needed little convincing to go to war with them in Sauron's name. They served as elite soldiers during the siege of Minas Tirith, but in the end the arrival of the Army of the Dead spelled defeat for Mordor and it's allies, including the Easterlings. After the war, it is likely that the Easterlings, like the other Men of Darkness, made peace with their old enemies and in exchange received Sauron's old lands as their own.