What is GIS?

A geographic information system (GIS) is a computer system designed to capture, store, manipulate, analyze, manage, and present all types of geographical data.  GIS can be thought of as a system that provides spatial data entry, management, retrieval, analysis, and visualization functions. GIS applications are tools that allow users to create interactive queries (user-created searches), analyze spatial information, edit data in maps, and present the results of all these operations. GIS is the science underlying geographic concepts, applications, and systems.

More information can be found at the following links:

Wiki

ESRI

ArcGIS