The simplest way of summarizing the poem is to paraphrase each stanza. Pay attention to the adjectives, what the speaker is saying and feeling, and consider any symbols.
In the first stanza, it is midnight. The speaker is "weak and weary" as he looks though an old book ("volume of forgotten lore"). He hears a tapping at his door. In the second stanza, he recalls that this was in December. So, it was cold and dark (midnight); a foreboding combination sets the mood. He is trying to read the book in order to take a break ("surcease") from the sorrow he is feeling from having lost Lenore, who has died.
In the third stanza, he is interrupted again by the curtains and he assumes it is some visitor. In the fourth stanza, he addresses this visitor and says that he has been napping. When he opens the door, he finds nothing but darkness. In the fifth stanza, he peers into the darkness. The theme of darkness is repeating and indicates a void, a sense of terror and of loss. He dares to whisper the name of his lost loved one, Lenore. All he hears is the echo of his own whisper.
In the sixth stanza, he hears something at his window and concludes that it is the wind. He is continually plagued by these random noises that have no explanation. There is a ghostly quality to this. It's as if he is being toyed with by a spirit. This is frustrating to him because he is trying to forget Lenore who is now, for all intents and purposes, a spirit/angel herself.
In the seventh stanza, the raven flies in and perches upon the Pallas bust, imposing his dark character upon this symbol of justice and reason. This symbolizes the notion that the raven, a symbol of darkness, is taking over for the speaker's reason.
In the next stanza, the speaker asks the raven for its name. The raven responds, "Nevermore." In the next stanza, he considers the significance of the name "Nevermore." In the next stanza, the speaker considers, aloud, that the bird will leave his room tomorrow, but the raven responds, "Nevermore." This implies that the raven, and all it represents (darkness, sorrow, etc.) will never leave the speaker.
In the next stanza, the speaker hopes and/or suggests that the bird keeps using this word because that's all it has been taught by some "unhappy master." In the next two stanzas, he tries to conceive of why the bird keeps using this word. By now, it is really messing with his mind. He sits "engaged in guessing" what "nevermore" could mean.
In the next stanza, he suggests that God and the angels have sent the raven to induce some forgetfulness in him, so that he might be relieved of his sorrow for his lost Lenore. But the raven responds, "Nevermore." In the next stanza, he asks the bird (perhaps sent by the Devil) if there is some balm (relief) for his sorrow. The raven repeats, "Nevermore." The speaker keeps asking if he will be relieved of his sorrow, knowing the raven will say no (nevermore). It's as if he is torturing himself, reveling in his own sorrow.
He continues torturing himself in the next stanza. He asks the raven if he ("this soul with sorrow laden") will be reunited with Lenore in heaven ("Aiden"). The raven says, "Nevermore." In the next stanza, he tells the raven to leave. The raven will not. In the final stanza, the speaker notes that the raven never leaves. Thus, he is always reminded that his Lenore is gone forever, never more to be seen again. The raven is a constant reminder of his sorrow.