El Nino is often discussed as a major component of the weather and climate, but it actually has it’s roots as an oceanic phenomenon. Strictly speaking, El Nino is the relative warming of the equatorial Pacific west from Peru and Ecuador. Below is an image which highlights the region which warms more than is typical.
This warming affects oceanic currents, which in turn affect atmospheric currents and weather patterns. Notably, the subtropical jet rides a little further north, which in the winter means that the southern US is far more active in the winter, in terms of precipitation. California often sees coastal flooding in El Nino years. The northern US, with the polar jet similarly forced northward, enjoys a warmer season than normal, while the precipitation can keep the southern US a bit more temperate.
The impacts are a bit more muted in the summer for the Northern Hemisphere, though the southern Hemisphere can tend to be warmer, as in South America, and dryer through Southeast Asia and Oceania.