Dunlendings were the ferocious, stunted and vicious men that lived in Dunland, close to Rohan. Also called the wild men of Dunland, they have long been enemies of the Rohirrim, although this hate is relatively baseless--it was the Numenoreans who had ruled them harshly in the past.
The Dunlendings were employed by the wizard Saruman to attack an raid the cites and settlements of Rohan. Since Grima Wormtongue had corrupted the mind of King Theoden, the Rohirrim were absent for the most part, and the farm boys and old men who picked up rusty swords proved to be little contest to the ferocious Dunlendings.
Wild Men were also present at the Battle of the Hornburg (Battle of Helm's Deep), as well as an odd breed of Half-Orcs, derived from Orcs and the Men of Dunland. They fought viciously against their old enemy, but when Gandalf stormed down the hill with a thousand Rohirrim at his back and his staff shining piercing light into the eyes of the Dunlendings, they dropped their weapons and surrendured. This proved to be a wise decision, as the Orcs who fled were killed by a massive forest of Hourns that blocked the entrance to the valley.
After Saruman's downfall, the Dunlendings retreated back into their homeland and did not trouble the people of Rohan. When the wizard Saruman attemtped to take over the Shire, there were a number of Dunlendings with him. However, they were slain or driven away by Hobbits, and when Saruman died at the hands of his own servent, Sauron, the basis of all evil, had fallen, and King Elessar took the throne of Gondor, the Dunlendings agreed never to trouble the free peoples of Middle Earth again, and their old and evil power was finally broken.