Warning: Wikipedia contains spoilers, but this article does not concern itself with the plot.
Heart of Darkness is a novella by Joseph Conrad. This highly symbolic story is actually a story within a story, or frame tale, narrated by Marlow to his fellow sailors as they lie becalmed in the London tidal estuary. It details an incident earlier in Marlow's life, a visit up the Congo River to investigate the work of Kurtz, a Belgian trader in ivory in the Congo Free State.
The theme of "darkness" from the title is reflected constantly within the book, in many different senses of the term. It is used to reflect the unknown (as Africa at the time was often called the "Dark Continent" by Europeans), the concept of the "darkness of barbarism" contrasted with the "light of civilization" (see white man's burden), and the "spiritual darkness" of several characters. This sense of darkness also lends itself to a related theme of obscurity - again, in various senses, reflecting the ambiguities in the work. Moral issues are not clear-cut; that which ought to be (in various senses) on the side of "light" is in fact mired in darkness, and so forth. Like Marlow himself, the astute reader emerges from the tale with an expanded comprehension of the darkness within his own mind.
Themes developed in the novella's more superficial levels include the naïveté of Europeans - particularly women - regarding the various forms of darkness in the Congo; the Belgian colonialists' abuse of the natives; and man's potential for two-facedness. The symbolic levels of the book expand on all of these in terms of a struggle between good and evil, not so much between people as within every major character's soul.