Voice. Plain and simple.
Just the other day someone was asking me about writing and voice.
When I teach introduction to writing, I start with these three singers. I have everyone listen to one song that all three do-- "Mack the Knife." (Lyle's version is "Moritat").
Each one of these singers has a very distinct voice. You know just by hearing a few notes who they are.
And the same goes for a writer's voice.
Some writers are so distinct with their style that readers familiar with their work will know them by their words.
Think about Star Wars, "Oh, the force is strong with that one." Now, replace the word force with voice.
Miss Marple is quite different than Hercule Poirot, yet they each have a strong voice. Miss Marple has a razor-sharp mind and Hercule has his "little grey cells" but I can picture each character and they are quite different. Christie used her voice to do this.
So voice has to do with the power of your character, the power that's inside of you as you write. That's why the adage, "Write what you know" is so important, and that's especially true for emerging writers.
Learning to bend my voice has been a challenge for me. What do I mean? Well, writing from a male perspective is one example. Too many times I've read through a student's work and had to ask, "Are you sure your character is a boy? Because I couldn't sense a penis." Needless to say, that produced some open mouths and gasps. Even in a middle-grade story, if the main character is a boy, he has to present as a boy. It doesn't mean the writer has to acknowledge sex organs, but they need to be there. And that's shown through voice.
A good strong voice.
Try this, just for fun.
Go to Itunes and listen to the sample of "Mack the Knife" by an assortment of singers. As you listen, write down a few words to describe the voice you hear.
I promise you, try this with five or six and by the end you'll have a new insight into the word, "voice."