El Niño is coming! But what is it?

The weather phenomenon is predicted to hit before the end of the year.

A UN agency has warned that the weather event, El Niño, will hit before the end of the year. The World Meteorological Organisation (WMO) - which is based in Geneva - has put the odds of El Niño occuring at 60% between June and August and at 75-80% from October to December.

But what is El Niño and should we be worried?