GIS stands for Geographical Information System. It is basically a map tied to a database. Any element of the map can be turned on or off to show trends in the database on the map.
In business, GIS can be used in several ways. Operational, Tactical and Strategic.
“Operational GIS applications include, for example, the use of GIS functionalities to monitor the provision of goods for a retail net. For this purpose, GIS will be used to support day-to-day routine activities. Tactical applications provide information needed for decision making. These tactical applications are generally used by middle management. Decision making requires the combination of all kinds of data relevant to the decision. GIS will support the combination of spatial data: for example, information on market potential and where competitors are located, to decide on the location of a new retail outlet. But tactical GIS use may also deal with issues like where to target a direct-mail advertising” — Fred Toppen & Hans Wapenaar
Some useful starting points.