From Webster's New World Dictionary:
tone (tõn) n
...2. a manner of expression showing a certain attitude
...15. a particular style or manner, as of writing or speech; mood: the macabre tone of Poe's stories.
A story without tone is like a serving of Pad Thai without any spices or flavoring. It's still there, it's still digestible, but it is all it can be? No.
Tone is the tool writers use to create mood and atmosphere and feeling. It is a hand that we use to guide the reader through our stories that allows them to feel what we want them to knew, to experience - as close as possible - the story they are reading without actually having to live the events.
Your tone can be dark, humorous, mysterious, whimsical...
To illustrate tone watch the following two videos. They are the same song, but performed by different artists to completely different effect.
As you know my now, they are both QUE SERA SERA. The first is Doris Day from the Alfred Hitchcock movie THE MAN WHO KNEW TOO MUCH. The second is from the Portland based band Pink Martini.
I'm actually partial to the Pink Martini version because I guess I'm drawn to things which are darker or a little askew. A very good friend of mine thinks the opposite. She likes the Doris Day version because, I think, of the lightness and the anything can happen (in a good way) feeling it envokes. Both create the tone the particular artist was going for. The same can be said for the prose you write or read. It's just another part of the writing tool kit.
Did you have a favorite? Or perhaps another example of Tone.