Théoden was the King of Rohan in the Lord of the Rings by J.R.R. Tolkien. Rohan, land of the Horse Lords, was to be taken over by Saruman while Sauron devoted his attention to the other lands and the finding of the Ring.
Saruman had Gríma Wormtongue placed as the King's assistant, and Gríma led the way to the fall of Théoden's mind. When all seemed lost, Gandalf and his fellowship members arrived to break Saruman's hold over Théoden, and to release the lands from the White Hand of Saruman.
Théoden, concerned for his people, decided to move his city to the walls of Helm's Deep. In the movie version, Gríma, knowing how the King thought, convinced Saruman to attack the caravan when it was vulnerable. Théoden and his kin, aided by the Fellowship members in attendance, drove back the attack, with the apparent loss of Aragorn as the price. In the book, a group of Rohirrim are attacked in the fords and are almost wiped out. The few left straggle back to warn the King.
When reading the book, one can see the weight of the crown on Théoden's head as he tried to make the best decision he could for his people. When all seemed lost, he rode out of the keep and drove the army of Saruman away with the help of a worn but still alive Aragorn, aided by Gandalf and the men of Rohan who were still loyal to Théoden.
Théoden was portrayed in the films by Bernard Hill.
Thanks to Swift^ for additional notes concerning the book versus the movie.