A geographic information system (GIS), or more commonly referred to as a geospatial information system is a system for capturing, storing, analyzing and managing data and associated attributes which are spatially referenced to the earth. In the strictest sense, it is a computer system capable of integrating, storing, editing, analyzing, sharing, and displaying geographically-referenced information. In a more generic sense, GIS is a tool that allows users to create interactive queries (user created searches), analyze the spatial information, edit data, and present the results of all these operations. Geographic information science is the science underlying the applications and systems, taught as a degree program by several universities.
Geographic information system technology can be used for scientific investigations, resource management, asset management, Environmental Impact Assessment, Urban planning, cartography, criminology, history, sales, marketing, and route planning. For example, a GIS might allow emergency planners to easily calculate emergency response times in the event of a natural disaster, a GIS might be used to find wetlands that need protection from pollution, or a GIS can be used by a company to find new potential customers similar to the ones they already have and project sales due to expanding into that market.
Open Source GIS Tools 
- GRASS - the most powerful opensource GIS application
- Natural Resources Database - GIS
- OSSIM - Open Source Software Image Map
- Quantum GIS - A user friendly GIS. An opensource alternative to ArcView.
- SAGA GIS
- Thuban - an Interactive Geographic Data Viewer
- TreeSap - Qualitative Reasoning GIS