The narrative technique used in "Heart of Darkness", is one of a story within a story. Although the primary narrator is Marlow, there is a second narrator, unnamed, who tells us about Marlow telling his story. There is also a third voice added to this narration which can be considered the author himself, who is really telling the whole story.
"Beyond these three dominant points of view are the individual viewpoints of the book's major characters. Each has a different perspective on Kurtz. These perspectives are often conflicting and are always open to a variety of interpretations."
Heart of Darkness uses a variety of narrative techniques that add complexity to the story. First, the story of Marlow's voyage into the heart of Africa and his discovery of Kurtz is told as a frame narrative--that is, a story within a story. The story opens aboard the Nellie, a cruising yawl on the Thames, and then Marlow begins to tell his tale about the Congo, which comprises the bulk of the story.
The other narrative technique in Heart of Darkness is the use of two narrators. While Marlow narrates the story of his discovery of Kurtz and his gradual but sure awareness of the horrors of what the European trading company is doing in Africa, the first narrator is an unnamed sailor about the Nellie. We don't find out who this person is, but it is this sailor who is listening to Marlow tell his tale. The reader only hears this narrator's voice at the beginning of the book and in the very last paragraph, when Marlow stops telling his tale. The only indication at the end of the book that the narrative has returned to the initial narrator is the phrase, "I raised my head." The "I" in this sentence refers to the first, unnamed narrator.
This "story within a story" technique is called a "frame story". Other examples of frame stories are "The Taming of the Shrew" and "The Princess Bride".