El Niño and La Niña are two faces of a large weather phenomenon. Here are a few questions and answers to help.
Q. What is El Niño?
A. El Niño is a climate phenomenon that involves the periodic warming of sea-surface temperatures across the central and east-central equatorial Pacific.
Every 3 to 7 years, the easterly trade winds in the Pacific weaken and allow the pool of warm water to drift from Australia to the western coast of South America, often triggering heavy rains there.
This vast pool of warm water is thought to set off a chain reaction that can affect jet stream and weather patterns around the world, especially in the winter months in the northern hemisphere. El Niño is sometimes referred to as ENSO for El Niño–Southern Oscillation. The Southern Oscillation is a seesaw of air pressures on the eastern and western halves of the Pacific. See El Nino Pattern chart .
Q. So, what is La Niña?
A. La Niña is essentially the opposite of El Niño. La Niña refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific that occurs every 3 to 5 years or so. The cool water can suppress rain-producing clouds, which leads to dry conditions. See La Nina Pattern chart .
Q. What do the terms "El Niño" and "La Niña" mean?
A. Peruvian fishermen first noticed the effects of a new El Niño at Christmas time, when storminess off the coast reduced the supply of fish. "El Niño" is Spanish for "the boy child," and is used to refer to the Baby Jesus. The name La Niña ("the girl child") was coined to deliberately represent the opposite of El Niño.
Q. How do these weather events usually affect the winter weather?
A. Because even the most dedicated scientists do not thoroughly understand El Niño and La Niña (we do not know, for instance, why the trade winds suddenly die down and allow the warm water pool to move eastward), we can only describe certain tendencies in the weather.
In the past, El Niño has often brought heavy rains to southern California and to a portion of the South from Atlanta to Cape Hatteras; it can bring relatively mild winter temperatures to the northern third of the country. However, these effects are not consistent in every El Niño event on record.
The stronger the La Niña, the more severe the droughts. The La Niña in 2009 is creating severe drought in much of the world, causing an agricultural crisis.
Links to More Information
To learn more about El Niño and La Niña, a good place to start is the Center for Ocean-Atmospheric Prediction Studies Library index of El Niño- and La Niña-related Web sites 
The National Oceanic and Atmospheric Administration (NOAA) has very detailed information about El Niño and La Niña: