The Franklin Institute Resources For Science Learning has written a summary on the basics of El Nino. 

Very well written, easily accessible and based on solid scientific knowledge.

“In the 1500s, fishermen who lived in South America began to wonder about a current of unusually warm water that came to their shore every few years near Christmastime. Since the fishermen believed in the birth of the Christ child at Christmas, and since they spoke Spanish, they named the hot water El Niño, which means “the infant” in Spanish.

Where do scientists look for El Niño? The hot water usually comes first to the coasts of Peru and Ecuador in South America.

But if we’ve known about El Niño for four hundred years, why is everyone talking so much about the hot water this year?
The 1997-1998 El Niño may or may not be stronger than ever before. Scientists are still deciding. One thing that is definitely different about this El Niño is the technology that scientists are using to study it.”

More on The Franklin Institute.

Photo from NASA.