The sweep strokes are the most common way to turn the boat.  The forward sweep is used to turn your boat while maintaining forward momentum.  This is far more useful than using a rudder and also provides extra stability.  When performing a sweep stroke, put your boat on edge on the side of the sweep stroke while keeping your upper body upright.  This greatly reduces the surface area of the boat on the water making it easier to turn the boat.  You start the forward sweep near your feet.  Instead of pulling the paddle blade straight back, you move the paddle blade in a wide arc away from the boat.  To avoid leaving the safety of the paddlers box, make your face and torso follow the paddle blade. As always, turn the boat with your torso rather than your arms.

This is an excellent video on the sweep stroke from Werner Paddles: The Sweep Strokes.

This is another good example of the sweep strokes from Mike Aronoff at CKAPCO: The Sweep Strokes.

The Stern Draw is a more advanced stroke.  It's commonly used in whitewater kayaking: Stern Draw.