Short answers, pulled from the story.
Denethor II is the 26th ruling Steward of Gondor in J. R. R. Tolkien's novel The Lord of the Rings. He is the son of Ecthelion II and the father of Boromir and Faramir. He dies by suicide in Minas Tirith during the Battle of the Pelennor Fields.
Denethor secretly used a palantír to observe Sauron's forces, but Sauron biased what he saw, showing the overwhelming might of Mordor while concealing Aragorn's fleet coming to Gondor's aid. When Faramir was carried back to the city apparently mortally wounded, Denethor lost all hope and ordered a funeral pyre in Rath Dínen. Gandalf rescued Faramir from the flames; Denethor died in them.
Tolkien scholar Jane Chance contrasts Denethor and Théoden as near-opposites. Théoden embodies what Tolkien called northern courage, the will to fight on despite certain defeat, and dies in battle. Denethor, by contrast, falls into despair and commits suicide during the same Battle of the Pelennor Fields. Tom Shippey identified a structural symmetry in their stories, noting that both are old rulers who have lost a son, both receive a hobbit's sworn allegiance, and both die at the same battle, but by opposite choices.
John Noble played Denethor in Peter Jackson's The Return of the King. The film portrays him far more negatively than Tolkien's novel, depicting him as greedy and self-indulgent. Critics including Tom Shippey objected to scenes such as Denethor gorging on food while Faramir rides out on a dangerous mission.
Denethor used Gondor's palantír to observe Sauron's strength, but Sauron controlled what Denethor could see, showing the scale of Mordor's forces and concealing any hope of rescue. Tolkien wrote that Sauron failed to dominate Denethor outright but made him believe his defeat was inevitable. The effort also aged Denethor rapidly.
William Conrad voiced Denethor in the Rankin/Bass animated adaptation of The Return of the King in 1980. Peter Vaughan played the character in the BBC Radio serialization of 1981.