Wikimedia
Wikipedia: Geographic Information System
Gives in-depth information about GIS - its history, what it is, techniques and tools, applications, etc.
University of Colorado
University of Colorado: The Geographer's Craft Project: Gis as an Integrating Technology
A comprehensive overview of GIS as a technology that brings different kinds of geographical data together to create something that is greater than the sum of its parts.
Other
Gis Lounge: What Is Gis?
Tries to give a clear answer to the often-asked question, 'What is GIS?'
Other
Esri What Is Gis?
This site is provided by ESRI, a leading GIS software developer. It explains how GIS works and ways in which it is used.
University of British Columbia
Ubc: What Is Gis?
This is a university-level introduction to GIS. It provides a comprehensive outline of important topics in this field of study.