In Chapter 3, Controlling the Lamp, the Camera, and Animating Objects, we discussed the use of Bézier Curves as F-Curves to control motion. Blender uses Bézier Curves to model as well. They are good to make objects with smooth curves.
One big difference between the curves used to control motion and the curves used to model is that the latter can move along the X, Y, and Z axes instead of just the timeline axis; therefore, they can be used with much more flexibility.