El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific, is one part of what's called the Southern Oscillation. The Southern Oscillation is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific; when the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific, and vice-versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the El Nino/Southern Oscillation or ENSO for short. South American fisherman have given this phenomenon the name El Nino, which is Spanish for "The Christ Child," because it comes about the time of the celebration of the birth of the Christ Child-Christmas.
To really understand the effects of an El Nino event, compare the normal conditions of the Pacific region and then see what happens during El Nino below.
Normal Conditions (Non El Nino)
El Nino Conditions
Scientists do not really understand how El Nino forms. It is believed that El Nino may have contributed to the 1993 Mississippi and 1995 California floods, drought conditions in South America, Africa and Australia. It is also believed that El Nino contributed to the lack of serious storms such as hurricanes in the North Atlantic which spared states like Florida from serious storm related damage.
Unfortunately not all El Nino's are the same nor does the atmosphere always react in the same way from one El Nino to another. This is why NASA's Earth scientists continue to take part in international efforts to understand El Nino events. Hopefully one day scientists will be able to provide sufficient warning so that we can be better prepared to deal with the damages and changes that El Nino causes in the weather.
http://kids.earth.nasa.gov/archive/nino/intro.html