Short answers, pulled from the story.
Heart of Darkness was first published as a three-part serial in Blackwood's Magazine in 1899. The novella received little attention upon its initial release and was not considered particularly notable by Conrad himself at that time.
Joseph Conrad's eight-month journey aboard a Belgian trading steamer on the Congo River in 1890 formed the backbone of the novella. Conrad served as a captain when the original captain fell ill and guided the vessel up the Lualaba River to the trading station Kindu in the Eastern Congo Free State.
The character Kurtz draws on multiple historical figures including Georges-Antoine Klein, Léon Rom, and a Belgian trader named Tshuma. Norman Sherry suggests Tshuma was the main model because he spoke three Congolese languages and was venerated by the Congolese to the point of deification.
Chinua Achebe delivered a scathing critique in his 1975 lecture titled An Image of Africa: Racism in Conrad's Heart of Darkness. He described the book as offensive and deplorable for dehumanizing Africans and depicting Africa as the antithesis of Europe and civilization.
Francis Ford Coppola released the film Apocalypse Now in 1979 which transposes the story from the Congo to Vietnam. The film stars Martin Sheen and Marlon Brando and has become one of the most influential adaptations of the novella.