PostGIS is an open source extension to PostgreSQL  that is often described as spatially enabling a PostgreSQL database. Because PostGIS understands complex geometries, it can be used not only to store spatial datasets, but also to query, summarize, and manipulate them based on spatial relationships. Using SQL statements, one can perform numerous common spatial operations such as overlays, joins, intersection, buffering, and much more. PostGIS can also serve as the backend database for powerful analysis and visualization applications like Quantum GIS , GRASS , and Mapserver .
Online tutorials and demos
- Excellent PostGIS tutorial materials  (FOSS4G workshop, 2007)
- Boston GIS , a compendium of tutorials for PostGIS and related applications
More on PostGIS
- Brief article on PostGIS  (December 2005), with links to additional resources
- The pgRouting Project : Provides routing functionality for PostGIS