Voice has two applications as a literary term: authorial and character voice. Both are distinct from anyone else's.
An author's voice is the distinct way he or she writes compared to all other authors. Writers' word choice, language use, and punctuation are all factors that express their voice. Authors convey a lot about their personalities and worldviews through voice. A strong voice establishes consistency and makes people want to continue reading.
"Adam was but human—this explains it all. He did not want the apple for the apple's sake, he wanted it only because it was forbidden. The mistake was in not forbidding the serpent; then he would have eaten the serpent."
Mark Twain is known for having a very strong authorial voice, as shown in this quotation from Pudd'nhead Wilson.
Character voice is how characters put their words together and discuss ideas. Well-developed character voice makes readers feel as though the character is a real person. To make a character's voice believable, writers try to make each character have a believable and consistent manner of speaking. Character voice is especially important for the character narrating the story (especially if the author is using first-person point-of-view).
"It kills me sometimes, how people die."
"I am haunted by humans."
"A small but noteworthy note. I've seen so many young men over the years who think they're running at other young men. They are not. They are running at me.”
Many people are drawn to Markus Zusak's book The Book Thief because of the strong character voice of its narrator, Death. Some of his quotes are above.