A GIS, or Geographic Information System, is a computer system that stores and uses data and aerial photography in a digital mapping environment.  GIS is unique in that it can perform analysis in a spatial (location-based) context – often through a process called overlay analysis.

GPS, or Global Positioning System, is a service that uses satellites and a special receiver to find your location based on the signals received from the satellites. GPS data is frequently used with GIS to create new data, capture data in the field, or to place a location within the context of a GIS.

GIS and GPS work together, but they are independent systems.