Historically in many cultures the raven is seen as a sign that evil and unpleasant events are about to occur. In The Raven the bird follows the unnamed narrator and has the ability to speak, the narrator finds this amusing rather than taking it seriously. There have been many theories as to what the raven in Edgar Allan Poe's poem represents, but most point to ones of sorrow and mourning for the loss of Lenore. Edgar Allan Poe allegedly said himself that the Raven represented the narrators "mournful and never-ending remembrance" of his lost Lenore.
herappleness's opening statement in her answer to your question caught my eye: "Historically in many cultures the raven is seen as a sign that evil and unpleasant events are about to occur." I'm sure this statement is true in some instances, with the raven appearing as a dire portent (e.g. in biblical references cited in one of the links below), but the raven has a lot of other associations. In Norse mythology, two ravens are the messengers and informants of Odin the All-Father, the ruler of the gods. In Native American mythologies of the Pacific Northwest, the raven is a trickster figure who generally is a friend of humans.
I can imagine how the raven might be a suitable symbol for loss because the bird often feeds (at least in stories) on corpses, particularly corpses on the battlefield.
As a side note, Edgar Allan Poe talks extensively in his short essay "The Philosophy of Composition" about his use of the bird symbol in the poem "The Raven." He even gives some interesting trivia, such as stating that he had initially thought of using a parrot!
The significance of Edgar Allen Poe actually stating that the bird in his Lenore work is a raven is that he had to use a bird that could 'talk.' Of course, no bird can actually converse with humans in a meaningful way, but it was enough to use a bird that could be taught to say a few 'words' as in his confused state, to the mourner in his half-dream state, this would have made sense. It would have been silly, for example, to use a robin or a swan. Ravens, as members of the crow family, are birds of high intelligence and can be taught to repeat sounds to which we humans attach significance, and to do tricks. I have read of Poe referring to a parrot also, and thought that this gaudy, comical bird with it's bright colors would have added completely the wrong tone.
There are many things to note about the Raven. First of all, the poem is about a loss, Lenore... who is nevermore. The raven is a black bird. The color black or darkness can represent evil, depression, grief, or sadness. In this case it appears the character nearly napping is in a crazed state of mind and this raven is the reminder of the pain the man experiences.
Birds are often used as symbols. The dove represents peace. Pigeons have been used to carry messages. The eagle represents freedom. This raven does the same thing, it plays the role of a haunting symbol of loss.