A GIS is a Geographic Information system. Basically, it's a map on steroids. Data is specifically tagged to location allowing very complex mapping and analysis, layer by layer. A layer of water features can interact with a layer of municipal services or lot lines or underground petrol tanks or vegetation cover – all of which allows true spatial analysis of interacting data points or phenomena.
We an look back to the start of GIS mapping in 1854, when London physician John Snow plotted cholera deaths on a London map with water pumps, allowing him to pinpoint the pump causing the disease outbreak. The first acknowledged GIS map was also the beginning of the study of Epidemiology and extended the concepts behind the Germ Theory of Disease.
NOTE: This page is still under development