El Nino is an occurence that happens every three to five years involving a reversal of the trade winds and the direction they are traveling. The trade winds normally proceed in a westerly direction, blowing warm, moisture rich air towards Australia and Indonesia. This air supplies the monsoon season that is so important for agriculture. The warm ocean water is replaced with cold, nutrient-rich water off the coast of South America. This water is very conducive to good fishing. Every three to five years there is a weakening in the air pressure that produces these trade winds, resulting in a negation or reversal of the wind direction. So the warm, moisture-rich air goes towards South America, depriving Indonesia of the monsoon rains, resulting in drought-like conditions. The fishing industry off the coast of South America suffers, as the warm water is not as nutrient-rich as the cold water is. Areas in South America and North America are bombarded with heavier than usual precipitation.
an abnormal warming of surface ocean waters in the eastern tropical Pacific,
It is said that its caused by natural occurrences, such as floods and droughts, that took place many years back. The source below has two simple diagrams that try to explain El Nino.