The Raven is perhaps Edger Allan Poe's most famous work. Written explicitly to appeal to audiences of the time, it made an immediate impression and although it never brought Poe fame and fortune directly, it has become a classic of the genre and and a staple of horror fiction, film, and music.
The titular Raven, which speaks the single word "Nevermore," come to bother a nameless narrator who tries to rid himself of it, but ultimately fails and must accept its presence.
The obvious symbolism is that of death: the narrator mourns his lost love, the beautiful Lenore, and the bird is possibly the most physical representation of the "black wings of death" present in myth and story. A raven is a carrion-eater, feasting on dead bodies, and in speaking the morbid and depressing word "Nevermore" allows the narrator to project his own depression and pain onto it. He could ask different questions or interpret the word in a different way, but the narrator's pain in such that he asks questions designed to further his cycle of self-loathing.
There are other symbolisms; one is the possibility of mental illness: in his misery, the narrator demands that the bird -- which is a bird -- answer his questions sensibly, and the bird responds in the only way it knows how. The narrator briefly entertains the truth:
Startled at the stillness broken by reply so aptly spoken,
“Doubtless,” said I, “what it utters is its only stock and store..."
but swiftly descends into madness, finally attributing all his personal ills to the bird and its supposed supernatural powers:
And his eyes have all the seeming of a demon’s that is dreaming,
And the lamp-light o’er him streaming throws his shadow on the floor;
And my soul from out that shadow that lies floating on the floor
Shall be lifted—nevermore!