A Geographic Information System, or GIS, is an organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information.
Or, in simple terms: A computer system capable of holding and using data describing places on the earth's surface.
Differences Between GIS and Other Systems
Many computer programs, such as spreadsheets, statistics packages, or drafting packages can handle simple geographic or spatial data, but this does not necessarily make them a GIS. A true GIS links spatial data with geographic information about a particular feature on the map. For example, the centerline that represents a road on a map doesn't tell you much about the road except its location. To find out the road's width or pavement type, you must query the database.
Using the information stored in the database, you could create a display symbolizing the roads according to the type of information that needs to be shown.
The Database Concept
In short, a GIS doesn't hold maps or pictures - it holds a database. The database concept is central to a GIS and is the main difference between a GIS and drafting or computer mapping systems, which can only produce a good graphic output. All contemporary geographic information systems incorporate a database management system.
A GIS gives you the ability to associate information with a feature on a map and to create new relationships that can determine the suitability of various sites for development, evaluate environmental impact, identify the best location for a new facility, and so on.