The Coriolis force is responsible for the spiraling of hurricanes and other large weather patterns. It also causes projectiles fired over long distances, such as artillery shells, to curve to one side. Pilots must also correct for the tendency of their flight paths to bend as a result of the Coriolis force.
Oddly enough, the Coriolis force is not a true force. It is what physicists call a fictional force. It is the apparent sideways force on an object moving across a rotating surface. In the case of hurricanes, cannon balls, and airplanes, it's the Earth's rotation that leads to the Coriolis force.
To get a feel for the origin of the force, imagine a boy and a girl on a playground carousel spinning counter-clockwise, with the boy standing at the edge and the girl standing at the very center. Now, suppose the girl in the center throws a ball toward her friend on the edge. In the time that the ball is in flight, the carousel has carried the boy a few feet to his right.
From the boy's perspective, the ball appears to have curved away and landed to his left. Of course, anyone on solid ground could tell that the ball moved in a straight line and that the boy moved away from it. That's why the Coriolis force is fictional; it looks as if a force is causing it to curve, but really the child's motion is just giving the appearance that the ball is curving.
In the Northern hemisphere, the Coriolis force deflects projectiles to the right and makes hurricanes spiral counter-clockwise. In the Southern hemisphere, things are reversed, with projectiles curving to the left and hurricanes spinning clockwise.
Although many people, including some misinformed scientists, believe that the Coriolis force determines the direction water rotates as it drains out of sinks and bathtubs, the force is much too tiny to affect the motion of water on such small scales. Instead, the way water drains in a sink has a lot more to do with the shape of the container and random currents in the water.
On the other hand, the Coriolis force can affect rivers. Riverbanks in the Northern hemisphere erode more rapidly on the right because of the right-handed Coriolis forces, while riverbanks in the Southern hemisphere erode more rapidly on the left.