Style in literature is the literary element that describes the ways that the author uses words — the author's word choice, sentence structure, figurative language, and sentence arrangement all work together to establish mood, images, and meaning in the text. Style describes how the author describes events, objects, and ideas.
One easy way to understand literary style is to think about fashion styles. Clothes can be formal and dressy, informal and casual, preppy, athletic, and so forth. Literary style is like the clothes that a text puts on. By analogy, the information underneath is like the person's body, and the specific words, structures, and arrangements that are used are like the clothes.