The writing style is the way in which the author conveys his thoughts. Syntax, word choice and imagery are a few of the many ways that a writer can convey his message. When trying to target a specific audience, an author may use a different type of writing.
In literature, style can be best described as the way an author writes, as well as the voice of that writer. One author's style is what is used to differentiate him from other writers. Writing style can change depending on the subject matter, but the voice used in writing is the writer's alone. Sentence structures, literary devices, and ideas, make each author unique.
Even as it may change and become influenced over time, the same basic style remains. In this sense, an author's writing style is unique.
Editors, on the other hand, use a different definition of style in literature. Style can also mean the mechanics of writing, as in how the author uses grammar and punctuation. The rules for this can change depending on whether the editor is using AP, MLA or another format.
Style also gives a writing substance, and breathes new life into an author's work. A bland article can be spiced up just by using different writing forms.