Every NURBS curve has a set of control points which control its shape but do not necessarily lie on the curve. The broken line joining the control points of a curve is called the control point polyline or polygon or defining polyline.
A numeric value, called the weight, is associated with each control point. Initially, the value of the weight is 1, but you may change it at any time. If you increase the weight of a control point, the curve moves closer to the control point, if you reduce it, the curve moves away from it. If you move the control points, the shape of the curve changes accordingly.