The way your character tells his story, the kind of language he uses and how he uses it to tell his particular story, is one way to think of voice. A lot of editors and agents say that the very first thing they look for in a manuscript is a strong voice.
I can see that. I love a strong voice as a reader. I start to believe in the story right away if I’m pulled in by the voice. Voice has to do with diction, of course; it has to do with our choice of words. But the way those words are arranged, the tone that emerges from those constructions, reveals character. I think that’s one of the reasons people react to novels with strong narrative voices. They feel an immediate connection to the character telling the story. They want to hear him say more, tell them more.