Apocalypse Now is a movie based off the plot of the Joseph Conrad's 1902 novella "Heart of Darkness". The movie was directed by Francis Ford Coppola and stars Martin Sheen as Captain Benjamin L. Willard and Marlon Brando as Colonel Kurtz. Although the book was set in the Congo, the film was made into a Nam movie and was therefore set in Vietnam and Cambodia during the war. The basic plot is that Willard is sent up river to kill the rogue Colonel, who has been operating without orders and out of U.S. military control for some time. Both the book "Heart of Darkness" and the film "Apocalypse Now" are very symbollic and offer unique interpretations on several themes, but rather than try to discuss all of these you should really just read/watch the book/movie and keep an open mind.
"Apocalypse Now" is a pretty crazy movie that makes a lot more sense if you read "Heart of Darkness" First.