It's a short story. One of the main reasons it's a classic is the exploration of the human mind. The narrator's struggle with his own sanity and his guilt is deeply psychological. Also, the way Poe builds suspense. He slowly reveals the narrator's plan to kill the old man, and then the aftermath of the murder. The vivid details like the 'dull blue eye' make the story very memorable.
The 'Tell Tale Heart' is a short story classic. Poe's use of language is key. His choice of words creates a sense of unease and foreboding. The pacing of the story is also perfect. It moves quickly from the introduction of the narrator's obsession to the actual murder and then to the final breakdown when the narrator confesses because of the sound of the heart. This well - crafted structure and the exploration of dark human nature are what make it a classic in the world of short stories.