Because scientists have to have complicated acronyms for everything, El Niño is more officially known as El Niño Southern Oscillation phenomenon or "ENSO" (atmospheric scientists call lunch the "Distribution of Mid-Day Nutrients and Caloric Energy" event).
ENSO is essentially a disruption of the atmosphere by warm waters in the tropical Pacific that has enormous consequences for the entire climate system and weather around the world. The current ENSO is one of the most drastic since 1997-1998 and is dumping record precipitation across the country.
This is largely due to a sudden surge in Ocean temperatures in the tropical Pacific around late October and November, currently ranging from 1.8 to 3.6 degrees Fahrenheit above normal ocean-surface temperatures.
Yet it's so important to keep in mind that the warming of the earth does not mean uniform warming across the board. There are so many factors that influence our complex climate system, that warming could (and likely will) mean that regions of the world will experience changing weather patterns in different ways, including colder winter weather and epic, record-shattering snow storms. This is perhaps what is most frightening about global warming: its incredibly difficult to predict where we're headed, and once we know, it's going to be too late to do much about it.