The Poe Decoder - "The Raven" for a discussion of the poem.
Ravens carry many symbolic associations: death, knowledge, good fortune, bad fortune.
They were the familiars of Odin, the Norse chief god, bringing him news of what was happening in the world. As scavengers, they are linked to death. In England, there is a belief that so long as there are ravens in the Tower of London, the country is safe.