What is an author's style?

An author's style is the author's unique combination of the components of language and writing.  Much the way great chefs create a meal using specialized ingredients, equipment, and techniques, authors draw on a wealth of knowledge about language to create effects and meaning on the written page.  

For example, read this excerpt from the first chapter of Jane Austin's novel Emma:

Emma Woodhouse, handsome, clever, and rich, with a comfortable home and happy disposition, seemed to unite some of the best blessings of existence; and had lived nearly twenty-one years in the world with very little to distress or vex her.

She was the youngest of the two daughters of a most affectionate, indulgent father; and had, in consequence of her sister's marriage, been mistress of his house from a very early period. Her mother had died too long ago for her to have more than an indistinct remembrance of her caresses; and her place had been supplied by an excellent woman as governess, who had fallen little short of a mother in affection.

Now, read this excerpt from Jane Austin's novel Sense and Sensibility:

The family of Dashwood had long been settled in Sussex. Their estate was large, and their residence was at Norland Park, in the centre of their property, where, for many generations, they had lived in so respectable a manner as to engage the general good opinion of their surrounding acquaintance. The late owner of this estate was a single man, who lived to a very advanced age, and who for many years of his life, had a constant companion and housekeeper in his sister. But her death, which happened ten years before his own, produced a great alteration in his home; for to supply her loss, he invited and received into his house the family of his nephew Mr. Henry Dashwood, the legal inheritor of the Norland estate, and the person to whom he intended to bequeath it. In the society of his nephew and niece, and their children, the old Gentleman's days were comfortably spent. His attachment to them all increased. The constant attention of Mr. and Mrs. Henry Dashwood to his wishes, which proceeded not merely from interest, but from goodness of heart, gave him every degree of solid comfort which his age could receive; and the cheerfulness of the children added a relish to his existence.

In both pieces, Austin's writing has the same general style.  She uses lengthy sentences with many dependent clauses, and she includes intricate details of her character's social lives and accounts of their relationships with the other characters.  Her diction (word choice) reveals that her stories concern people of status and wealth - inheritor, estate, wealth, bequeath, generations, gentleman, comfortable.

When considering an author's style, you can think about any of these components of composition.

Subject Matter

Selection of Detail

Point of View

Diction

Figurative Language /Imagery

Attitude

Tone

Pacing / syntax

Organization