In the Adobe Illustrator manual, which has not really changed much at all since version 1.0 was published in 1987, a specific procedure is detailed to draw curves:

Drawing the first point in a curve is simply made out: A. Positioning the Pen tool B. Starting to drag (while keeping the mouse button pressed down) C. Dragging to extend direction lines

Position the Pen tool where you want the curve segment to end, and do one of the following: To create a C‑shaped curve, drag in a direction opposite to the previous direction line. Then release the mouse button.

To create an S‑shaped curve, drag in the same direction as the previous direction line. Then release the mouse button.

Excerpt from Illustrator User Guide - Drawing - Draw with the Pen, Curvature, or Pencil tool - http://help.adobe.com/en_US/illustrator/cs/using/WS3f28b00cc50711d9-2cf89fa2133b344d448-8000.html#WS55B8A5DC-5496-494d-ADA8-FFE0FA5DAEA1