In "The Raven," by Edgar Allan Poe, how do you explain the raven and its visit?

First in answer to this question, I would say that the Raven does not really need to be explained. The poem is an example of The Gothic, a genre that features fantastical elements much more incredible than this talking bird. For example, Gothic stories or novels can feature ghosts, vampires, organs that play themselves in the middle of the night, and other such phenomena. However, if one wanted to explain the Raven as a natural...

First in answer to this question, I would say that the Raven does not really need to be explained. The poem is an example of The Gothic, a genre that features fantastical elements much more incredible than this talking bird. For example, Gothic stories or novels can feature ghosts, vampires, organs that play themselves in the middle of the night, and other such phenomena. However, if one wanted to explain the Raven as a natural rather than supernatural phenomenon, I think there is evidence in the poem to do so. The narrator himself comes up with an explanation that is quite logical:

  “Doubtless,” said I, “what it utters is its only stock and store

Caught from some unhappy master whom unmerciful Disaster

   Followed fast and followed faster till his songs one burden bore—

Till the dirges of his Hope that melancholy burden bore

            Of ‘Never—nevermore.'”

In other words, the previous owner of the Raven was someone who had so many disasters in his life that he was continuously exclaiming, "Nevermore!" The bird picked up this one word from its master, like a parrot that learns to imitate the words it hears most often. Can ravens "speak" in the same way parrots can? Yes, absolutely, as you can see from the links below. 

The narrator's problem is that he asks the wrong questions. He keeps asking questions that, when they produce an answer of "Nevermore," plunge him into deeper depression about his lost Lenore. 

What could explain the Raven's visit? That could simply be a random fluke. Or, since animals can often sense human emotions, the Raven may have gotten lost from its master and mistook the narrator for his master because the feeling of despair emanating from the narrator was so similar to the mood of his master. That would mean the narrator's depression was so deep it was "palpable" even through his "window lattice." Given how easily he falls under the spell of the Raven's shadow, that does not require too great a stretch of the imagination.