Most forms of conventional writing are prose. Prose consists of grammatical sentences, organized in paragraph form. It features a natural flow of speech and no patterns of rhythm or rhyme. It is not written in verse, which is what you find in poetry. Verse takes many forms, but it sometimes looks like these initial lines of Robert Frost's poem "Stopping by Woods on a Snowy Evening":
Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
The definition of prose is pretty wide, and it therefore takes many different forms. Here are some types of works that you likely encounter frequently and could classify as prose:
Novels and short stories: Most typical required reading in your English courses, from The Story of an Hour to Harry Potter to Pride and Prejudice, are prose. These works of fiction vary greatly in voice, vocabulary, and form, but they all all ultimately follow typical speech patterns and sentence formations. Whether you read science fiction, metafiction, romances, or historical novels, you are enjoying a work of prose.
Textbooks and biographical publications: You likely use a historical textbook for your history class and may pick up a book on Guion Stewart Bluford, Jr. for a research project in your astronomy class. Both of these publications are written using typical grammatical forms and structures, with recognizable paragraphs on the pages. This is prose.
Scientific writing: Scientific writing may explain how the world around us works, investigate theories, and provide the outcomes of scientific experimentations. These works are written in prose using typical grammatical structures.
Business writing: Companies need to effectively communicate their processes and work flow both within the company and to their prospective clients. They utilize prose writing for effective communication, often employing forms such as emails, memos, agendas, and letters to convey needed information.