A chair is a piece of furniture for one person to sit on, with a support for the person's back. When a chair is a very simple one, you say that someone sits on it.
Anne was sitting on an upright chair.
Sit on this chair, please.
When a chair is a comfortable one, you usually say that someone sits in it.
He leaned back in his chair and looked out of the window.
Useful english dictionary. 2012.