A short story is usually written in fictional prose. Fictional writing can be defined as any narrative that is not based in fact. Prose language is standard written language, the same sort of writing you would use in a letter to a friend. The word "prosaic" can be used to refer to the dull or commonplace, but that does not mean prose language has to be boring, as the range of great short stories written through the ages proves. As the name suggests, a short story is far shorter and less involved than a full-length novel and might, for example, involve the setup and resolution of a specific situation, rather than the detailed exploration of characters that is common in a novel.
The word "poetry" comes from the ancient Greek verb "to create." While a short story is usually written in recognizable sentences, a poem is composed of lines, which may not observe the usual grammatical rules. Furthermore, rather than being organized into paragraphs, a poem is made up of verses, or stanzas. Laid out on a page, a poem will not resemble the prose used in a novel or short story. In a poem, words are often chosen not just for their functional, prose meaning, but for their sound, or feel.
Example of the Beginning of a Short Story
"In the old days Hortons Bay was a lumbering town. No one who lived in it was out of sound of the big saws in the mill by the lake. Then one year there were no more logs to make lumber." -- "The End of Something" by Ernest Hemingway
Example of the Beginning of a Poem
April is the cruellest month, breeding Lilacs out of the dead land, mixing Memory and desire, stirring Dull roots with spring rain. Winter kept us warm, covering Earth in forgetful snow, feeding A little life with dried tubers. -- "The Waste Land" by T.S. Eliot