Since body gestures are also part of communication, Indians move their head sideways generally to disagree with anything. Simply put, it means No. if the movement of head is done vertically then it means "yes". it is funny but very common down here :)

One of the most important gesture is pressing both hands and bowing a little and saying Namesty. We do this while saying hello

Indians shake thier heads while talking because in indian culture you have to be more responsive and you have to show this in your way of talking. Indians don't just move their head in agree and disagree but while having a general conversation also. General example = main aapka kaam kr dunga ( i will do your work ). Most of indians move their head upwards down and it is not just showing agreement but giving the person confidence to believe in your words.