Prose is such a common thing that we rarely even think about it, much less define it, and when we do, we tend to think about what it is not. Prose is not, for instance, poetry. It does not have lines and stanzas that follow a metrical pattern (like poetry does). What prose does have is a flow of sentences and paragraphs that generally follows the natural grammatical and syntactic flow of spoken language. This paragraph is a prime example of prose.
Prose appears in several different categories. There is nonfictional prose as we read in textbooks, works of history, or the newspaper. There is the fictional prose of novels and short stories. There is even heroic prose that was once passed down orally but is now recorded as legends.
Prose allows writers to communicate with their readers in straightforward language that can even be conversational, yet it is also flexible enough to allow for high-level vocabulary, complicated syntax, and the expression of complex ideas. Prose allows authors to build plots and characters, to express descriptions in vivid detail, and to let their imaginations run so that their readers can follow them into new worlds and new experiences.
While poetry has its own beauties and appeal (as well as its own potential for creative expression), many writers choose prose as their tool for self-expression no matter what their genre.