Rohan is a kingdom of Middle-earth situated east of the Misty Mountains, west of the river Anduin, north of Gondor, and north-west of Mordor, in a region formerly part of Gondor (which was called Calenardhon when it was a part of Gondor). Rohan is also referred to as the Riddermark or just the Mark. It is a land of rolling grassland, known particularly for its fine horses. Gandalf's steed, Shadowfax, comes from Rohan. Its capitol is Edoras, the site of Meduseld, the Golden Hall.
The people of Rohan, known as the Rohirrim, Riders, or Eorlingas, are based more or less on the Anglo-Saxon and Norse peoples of the Real World. They are a proud people, mainly horsemen and farmers. They tend to be very blond. Their language, Rohirric, is an old dialect of Westron (and is represented by Old English in the text of The Lord of the Rings). They originally lived in the plains of Rhovanion east of Mirkwood, then moved to the upper vales of the Anduin when the Wainriders invaded from the east, until they rode south to the aid of Gondor in the Battle of the Field of Celebrant. As a reward for their services, the Steward at the time (Cirion), granted the land of Calenardhon to the Rohirrim to be theirs.
During the War of the Ring, the King of Rohan was Théoden. He was succeeded by his nephew, Éomer.
The class plural for people from Rohan is "the Rohirrim" (singular "Rohir"). Use of either "Rohans", "Rohirrics", or similar nonsense to describe more than one Rider is a charge, and depending on the agent may provoke murderous rage.