What is GIS?
GIS is a system of hardware and software used for storage, analysis, and mapping of spatial data. In a general sense, the term describes any information system that integrates, stores, edits, analyzes, shares, and displays geographic information for decision making. GIS applications are tools that allow users to search spatial databases, analyze spatial information, edit data in maps, and present the results of all these operations. In a GIS spatial features are stored in a coordinate system (latitude/longitude, State Plane Northing & Easting, UTM, etc.), which is an address to a location on the earth. Spatial Features also have nominal, ordinal, interval, or ratio data that is associated with them. GIS can be used for city planning, resource management, archeological site work, traffic planning, accident investigation, and much more.
ESRI "What Is GIS" is an excellent page for GIS fundementals.
Texas A&M Corpus Christi Conrad Blucher Institute of Surveying Sciences Web Page.