Here are two good introductions to GIS. The first is a series of videos from Pennsylvania State University called Geospatial Revolution. This four-episode series explores how geospatial technologies, including GIS, are changing our world for the better, and how new jobs are being created in this industry.
Another great text-based and interactive resource is called GIS.com. This is a more detailed introductory look at what GIS is, and how it fits within the larger geospatial world. This site includes interactive demos that allow you to solve some real-world problems using GIS.