El Nino is the positive phase of the El Niño/La Niña-Southern Oscillation, which is when the Pacific Ocean warms in the tropical regions. This warming has major impacts on the jet stream and weather patterns around the world. The impacts on weather are related to the strength of the El Nino. A strong El Nino can cause massive Pacific storms that bring flooding rains to certain areas like the southwestern U.S. and plentiful rain to normally dry western South America. It will also bring drought conditions to the Western Pacific and Australia. A strong El Nino even impacts the Atlantic hurricane season by making high level winds stronger than normal in the tropical Atlantic, which makes it harder for hurricanes to form.
El Nino is a natural occurance. The Pacific is usually either in an El Nino or in a La Nina (which is when the Pacific is cooler than usual in the tropical regions).