http://www.gisdevelopment.net/tutorials/tuman002b.htm
Geographic Information System (GIS) is defined as an information system that is used to input, store, retrieve, manipulate, analyze and output geographically referenced data or geospatial data, in order to support decision making for planning and management of land use, natural resources, environment, transportation, urban facilities, and other administrative records.
The key components of GIS are a computer system, geospatial data and users, as shown in Figure 1.1.
A computer system for GIS consists of hardware, software and procedures designed to support the data capture, processing, analysis, modeling and display of geospatial data.
The sources of geospatial data are digitized maps, aerial photographs, satellite images, statistical tables and other related documents.
Geospatial data are classified into graphic data (or called geometric data) and attributes (or called thematic data) as shown in Figure 1.2. Graphic data has three elements ; point (or called node), line (or called arc) and area (or called polygon) in either vector or raster form which represent a geometry of topology, size, shape, position and orientation.
The roles of the user are to select pertinent information, to set necessary standards, to design cost-efficient updating schemes, to analyze GIS outputs for relevant purpose and plan the implementation.