Originally from the East of Rhûn, The Wainriders were named from the large, horse-drawn wains (wagons) they used. They were at war with Gondor during the years 1856 through 1944 of the Third Age.

After the Great Plague, Gondor’s enemies from the east were quiet for a time. But after 200 years had passed, the Wainriders returned and were involved in war with Gondor for nearly 100 years. In TA 1856 they came from Rhûn and, battling Gondor and its allies, they were victorious, defeating Gondor's army and slaying King Narmacil II in battle. They then took control of the lands of Rhovanion, enslaving all who lived there.

In TA 1899 the enslaved peoples rebelled, aided by Gondor. King Calimehtar, son of Narmacil II, sought to avenge his father’s death. He received messages from Marhwini of the Éothéod stating that the Northmen were planning a revolt against those who oppressed them. Along with those messages came some telling the king that the Wainriders were also preparing to attack Gondor. The armies of Gondor, with help from the Éothéod and those Northmen who revolted, were able to defeat the Wainriders for a time on the plain of Dagorlad, thanks in part to the early warnings from the people of Marhwini.

In 1944 the Wainriders attacked again, this time aided by the Haradrim and the Nazgûl. Gondor was attacked simultaneously from the East and the South and had to divide its forces to meet both threats. The Eastern army was defeated by the Wainriders and King Ondoher and both his sons fell in battle. The Southern army of Gondor however, defeated the Haradrim. Then, heading East, they surprised the victorious Wainrider forces, defeating them in the Battle of the Camp and decimating them. The Wainrider survivors fled into the Dead Marshes and perished.

It was some time later discovered that this invasion of the Wainriders and their allies was orchestrated in part by Sauron. The Dark Lord had used the time that Gondor had been forced to relinquish their lands to the east of Anduin to open the gates of Mordor and allow the Ringwraiths back into his land.

[additional information from Nienna-of-the-Valar]
Encyclopedia entry originally written by PotbellyHairyfoot