Definition of Style for Creative Writers
Style, to a fiction writer, is basically the way you write, as opposed to what you write about (though the two things are definitely linked). It results from things like word choice, tone, and syntax. It's the voice readers "hear" when they read your work.
The Voice That Is Uniquely Yours
Naturally, your writing style will change depending on your subject matter and the point of view. However, when we talk about developing your writing style, we mean the voice that is uniquely yours. That voice will change as your writing develops, of course, but like personality, the foundation is already there.
The Mechanics of Writing
To an editor, on the other hand, style refers to the mechanics of writing, i.e., grammar and punctuation. These rules change depending on what field you're in. For instance, according to Chicago style, used by book publishers, book titles are italicized. Reporters, using Associated Press (AP) style, would put the same title in quotation marks. (Literature students use MLA style, which also italicizes book titles.)
Examples: His style of writing had been heavily influenced by Hemingway: he wrote in simple, direct sentences and used few adjectives.