[One Axe to Rule them All]

I always imagined them as a very tribal and fierce people, accustomed to living in the wild and fighting to keep what they had, everything they had was portable, and small wooden chariots were their largest tools of combat, and they fought in a mongolish style, seiging tows and taking what they wanted. They fought with spears, scythes, axes, and compound bows, and I always imaged them as a dark-skinned people with short beards(I know it's not very realistic) full of black hair.

Then again, just my imagination...

*EDIT*
I found a rough description on the encyclopedia of Arda

A confederation of Easterlings that descended upon Gondor and its allies, some two centuries after the time of the Great Plague, and remained a dire threat to the South-kingdom for nearly a hundred years. The Wainriders, who took their name from the wagons and chariots in which they rode to war, quickly overran Rhovanion and soon came to battle with Gondor itself. Narmacil II was lost in battle, and for a time all of Gondor's possessions east of Anduin were lost to it. Narmacil's son Calimehtar won a temporary reprieve when he defeated the Wainriders on the plain of Dagorlad.

After that defeat, the Wainriders planned a crushing revenge. Allying themselves with the Men of Harad and Khand, they orchestrated a simultaneous assault on Gondor from the north and the south, and the South-kingdom came close to destruction. King Ondoher was lost in the northern battles, but his general Eärnil defeated the southern invaders. The victorious Wainrider army in the north, celebrating their victory over Ondoher, suddenly found themselves set upon by Eärnil. In an encounter that became known as the Battle of the Camp, the Wainriders were put to rout, and those that survived fled back to their eastern domains.

It was later seen that the invasions of the Wainriders, as so many of Gondor's perils, were engineered by the emissaries of Sauron. While Gondor's control of the lands east of the Anduin had been lost, the borders of Mordor had been open, and during this time the Nazgûl had taken advantage of Gondor's plight to re-enter the land of their master.