Marshal of the Mark was a title of high rank within the army of Rohan. The Marshals were considered lieutenants of the King, and commanded not only a fully equipped Éored, but were charged with one-third of all the Muster of Rohan. As such, there were usually three Marshals.
The first Marshal's charge was the King's lands. These included Folde and Harrowdale, and the muster also of Edoras, the capital. The second Marshal was usually charged with the West-Mark, including the Fords of Isen, a pivotal location in Rohan's defense during the War of the Ring. The Hornburg (Helm's Deep) was the base of the West-Mark's Marshal. The East-Mark often went to the third Marshal. His headquarters were at Aldburg, once the home of Eorl the Young, first King of the Mark. The second and third Marshals, however, were interchangeable, and were assigned based on need by the King.
At the time of the War of the Ring, Théoden King son of Thengel still held loosely the title of first Marshal. During his youth, he had enjoyed leading the Muster of Edoras, but as he fell under the growing shadow of Saruman, and his servant Wormtongue, Théoden began to neglect his charges and duties as a Marshal. The second Marshal and commander of the Westfold was Théodred, son and heir of Théoden. Éomer son of Éomund, nephew of the King, was the third Marshal, and filled his father's post as commander of the East-Mark. Due to Théoden's condition, the de facto commander at Edoras was Elfhelm, who acted as a Marshal of the Mark. Háma, the loyal captain of the King's guard, was also at the capital.
When war with Saruman began, Théodred assumed command of Rohan's army, despite the orders of his father speaking through his wicked counsellor Gríma. Théodred ordered a muster to Edoras and soon led a good portion of the King's riders west to strengthen the Fords of Isen against invasion. Later, Éomer defied the King and led his Éored (riders of his own household, his personal guard) to remove the herdfolk of the East-Mark behind the fens of Entwash. There he encountered the Orcs returning to Isengard from the Falls of Rauros, and later Aragorn, Legolas and Gimli.
During the battles of the Lord of the Rings, the renewed Théoden assumed command of the army. With Théodred slain, Éomer acted as first Marshal. Elfhelm led the muster of Eastfold, while Grimbold, previously a captain and hero of the battles at the Fords of Isen, commanded the riders of Westfold. After the War of the Ring was ended, Éomer King reordered the positions of the Marshals. He himself commanded the Muster of Rohan, and caused the position of first Marshal to be held ever by the King or, in the case of age or sickness, the King's heir (assuming the King had at least another clear claimant to the throne). The positions of second and third Marshal were largely dissolved, replaced by the titles Marshal of the East-Mark and of the West-Mark.