On the global scale, prevailing winds are divided into three main categories: the northeast and southeast trade winds, the westerlies and the polar easterlies. Their varying locations and directions come from how the earth rotates and how the sun heats the atmosphere. For example, the sun's rays hit the equator most directly and thus, steadily heat the atmosphere there, which leads to a continuous rise of warm air around the equator.
The rising air at the equator gradually drifts north and south, where it eventually cools, sinks and, because of the convection effect, travels back to the equator. This continuous circulation of air -- the trade winds -- was used by ancient sailors to sail from Europe to the Americas since the trade winds blow from east to west. The prevailing westerly winds, because they're warmer than polar air, are drawn to the poles. Conversely, the colder, polar easterly winds seek warmer atmosphere and blow away from the poles. This basic pattern of heating and cooling and the convection effect form the earth's dominant winds.
Designations such as "easterly" or "westerly" depend on which direction the wind blows from. If it blows from the west, it's westerly; if it blows from the east, it's easterly. The polar easterlies in particular can have a significant effect on weather. As the cooler polar air moves toward drastically warmer air to the north or south, powerful and violent storms may result. On a smaller scale, dominant winds can change the appearance of landscape in a particular spot. In deserts, you can tell where the dominant wind blows by observing the dunes. Linear dunes, for example, are long, straight and ridge-shaped, and stretch parallel to dominant wind direction.
In addition to the trade winds influencing the historical sailing trade, dominant winds also have modern-day effects on society. Designers ensure that airport runways align with dominant winds to aid landings and takeoffs. City planners account for prevailing winds by placing dumps, factories and sewage disposal plants so that pollutants don't drift toward the city's population. Even individual homeowners work with prevailing winds for a better living experience. In the Northeastern U.S., the dominant winter winds blow from the northwest. Smart homebuilders, therefore, will have few, if any, windows on the northwest sides of their homes to keep the cold winds from sneaking indoors.