According to About.com, there are many causes for drought, but the most important one "relates to the amount of water vapor in the atmosphere, as this is what creates precipitation." Moist, low pressure systems tend to bring more rain, snow, sleet and hail, while dry high pressure systems lead to a lack of precipitation.
According to HowStuffWorks, the National Weather Service has four operational definitions of drought. Meteorological drought involves typical seasonal and regional patterns and is often not apparent to the naked eye. Agricultural drought begins to affect local crops. Hydrological drought causes a drop in surface as well as subsurface water supplies, causing lakes, streams, groundwater and reservoir levels to fall. In socioeconomic drought, the dryness escalates to the point that it affects everyone, causing food shortages, a decrease in tourism and the displacement of populations. The effects of drought are more severe in underdeveloped parts of the world. They lead to disease, starvation, political unrest and even armed conflict.
Prolonged drought has long-term effects in terms of soil erosion. In 1930, a lack of precipitation in the Great Plains led to the Dust Bowl. Reference.com states that this drought eventually affected approximately 50 million acres.
Many factors increase the chances of drought. Deforestation and global warming are two man-made factors affecting available moisture. However, winds and climate changes also play a role in the devastation drought brings.