A geographic information system (GIS) is a system used to describe and characterize the earth and other geographies for the purpose of visualizing and analyzing geographically referenced information.

Many have characterized GIS as one of the most powerful of all information technologies because it focuses on integrating knowledge from multiple sources (for example, as layers within a map) and creates a crosscutting environment for collaboration. In addition, GIS is attractive to most people who encounter it because it is both intuitive and cognitive. It combines a powerful visualization environment—using maps to communicate and visualize—with a strong analytic and modeling framework that is rooted in the science of geography.

This combination has resulted in a technology that is science based, trusted, and easily communicated using maps and other geographic views.
(http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Introduction/00v200000002000000/)