Wind is the name we give to moving air. Differences in air pressure can cause air to move. Air pressure is caused by the fact that air has weight and is being pulled downward by the earth's gravity. Although a small volume of air has little weight, the weight of all the air in the atmosphere being pulled down by gravity causes a pressure of nearly 15 pounds per square inch (p.s.i.) on the earth's surface.
The major wind patterns on Earth are caused by two factors:
Because the earth is generally hotter near the equator, air there is less dense and rises up. Because the polar regions are colder the air there is denser and sinks. This creates giant convection currents in the atmosphere. The region between the equator and each pole is really too big to have one giant convection current. As shown by the diagram below, each of these areas is actually broken up into 3 smaller convection currents.
Because the earth is rotating on its axis the winds at the equator are deflected to the west. This is called the Coriolis Effect. This also causes winds in mid-latitudes (where we live) to deflect to the east. In the polar regions, winds deflect to the west.