"A geographic information system (GIS) lets us visualize, question, analyze, interpret, and understand data in many ways that reveal relationships, patterns, and trends in the form of maps, globes, reports, and charts". -ESRI
To get a full description of GIS and GIS at Saddleback College - visit Professor Laszlo's GIS Saddleback College GIS web pages here.
For a thorough explanation and samples - a good source is ESRI's GIS page at:
Also - check the Links page at left for many other sources and information about GIS.