A description is given of the origin of the terminology and the nature of the phenomenon known as El Nino.  El Nino (EN) refers to a substantial warming of the tropical Pacific Ocean, and it is linked with changes in the atmospheric circulation and pressure patterns known as the "Southern Oscillation" (SO). Together scientists refer to the whole phenomenon as ENSO. La Nina is the cold phase of ENSO.  A description is also given of the teleconnections around the world linking droughts and floods and temperature anomalies to ENSO and the physical understanding of the whole phenomenon. An observing system has been implemented to track El Nino evolution, especially through buoys moored to the bottom of the ocean that telemeter information by satellite, so it is available almost instantaneously over the world  wide web. Tracking subsurface ocean temperatures allows predictions to be made and examples will be given.