Coriolis force

Wind is doesn't follow a straight path from high pressure systems to low pressure systems.
When you stand with your back to the wind direction, wind is deflected to the right on the Northern Hemisphere, and to the left on the Southern Hemisphere.
This phenomenon is caused by the rotation of the earth and is called the Coriolis force, after the Frenchman Gustave-Gaspard de Coriolis (1792-1843) who discovered it in 1835.

The Coriolis force is directly related to speed and latitude. Stronger winds are deflected more than slower winds. Winds at the poles are deflected more than winds near the equator. There isn't any Coriolis force at the equator. Because wind accelerates it gets more deflected as its speed gets higher, which explains the curving path of wind.
This curve can become a circle moving around a pressure center, which are clockwise around high pressure centers on the Northern Hemisphere. This can be a geostrophic wind or a gradient wind.

Example
The Coriolis force is hard to understand, that's why we will try to explain it with an example:
Imagine you are in a car, driving on the freeway. If you would throw something out of the window (never do this!), the object would go away from the car, but it would also move to the right.
Another example, which you can try at home. You will need an old lp and a record player. Draw a line with a pencil from the center of the spinning record, and move it straight towards the outside. You will notice that the line will be a spiral. When the record spins harder, there will be more circles.