@Craven de Kere,
What prince wrote is correct....
1) Nodding (moving up and down) is a YES
2) Shaking (moving side to side) is either a no, or I am not so sure, or I dont believe that you are saying this, or I think u r a moron.....
Apart from these two, there is another side-to-side movement. I dont know how to explain the movement in words - You do not turn your face/head to the side like you would in 2). Instead, you bend/bob your head on each side of the shoulder (usually at a 45 degree angle). This third head-bobbing movement means "Okay".
So, if you were to ask me "Are you an Indian". I would use 1) to denote yes. I would not use 3)
If you ask me "Do you eat meat?". I would use 2) to say no (because I don;t).
If you asked me "I'm going shopping. Will you come along with me?". I would use 3) to indicate "okay, i'll come along"....