The trade winds are caused by a combination of convection air currents and the Earth's rotation. Air is warmed near the Equator and moves towards each pole, respectively. This air is deflected by the Coriolis effect, or the spin of the Earth, causing it to fall back towards the Equator in both hemispheres.
As air warmed at the Equator rises towards the poles, the rotation of the Earth causes it to deflect and flow back towards the Equator. In the northern hemisphere, the winds blow from the east to the west, while they blow in the opposite direction in the southern hemisphere. These winds tend to be much stronger over open water than they are across land, which has made them ideal for sailors.
The trade winds have been used for many centuries to decrease the transit time of ships traveling around the world. The trade winds were instrumental in the settling of the Americas because they reduced the long trip from Europe by many days or, in some cases, by weeks. These winds blow nearly constantly and always in the same direction, and since the trade winds are always blowing across the surface of the oceans, they also have an effect on ocean currents.