The horror of being; the darkest depths of man's soul; the deepest fears brought about by darkness: it's all here. This is the work of the original genius of terror. And the most terrifying thing about Poe's stories and poems is that the threat doesn't come from a monster, or a devil, or a murderer: it comes from inside yourself, from your mind and your heart. There's no escaping them. Poe is not, of course a "terror" writer. He's just a writer, and one of the best there has been. His work can not be confined to a "genre". His tales touch horror, but there are some analytical, metaphysical, futurists, and tales of love (strange love, but love).
As correctly pointed out by other reviewers, Poe practically invented the mystery tale in which the detective is an amateur who solves the problem through reason and deduction alone ("The crimes of the Rue Morgue"). A wonderful cryptic and deductive tale is "The golden bug". "The cask of Amontillado" is a masterpiece of cruel vengeance. "The pit and the pendulum" is pure terror, like "The black cat".
The poems have even more variety. You know what the famous ones are: The Raven, The bells, Annabel Lee. Here, the most remarkable characteristics are music and rhythm. "Quoth the raven: nevermore!", and the ringing of the bells, the bells, bells, bells, etc. My personal favorite is Annabel Lee, but there are many other, less known, which are just excellent.
Poe was a troubled man, addicted to drugs and alcohol, who died in a miserable way (some thugs made him drink to use him in an electoral fraud; he died from drunkness on the streets of Baltimore). But his intellect and sensibility (hypersensibility) made him a true genius, a profound connoiseur of the human soul, up and down. His writing is superb and he will remain as a master of literature for centuries to come. In case you have never approached his work, do so now. Choose your favorite couch; wait until everybody is asleep, get yourself a good drink, and travel to the bottom of your own soul.