From their website: PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS “spatially enables” the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS), much like ESRI’s SDE or Oracle’s Spatial extension. PostGIS follows the OpenGIS “Simple Features Specification for SQL” and will be submitted for conformance testing at version 1.0.
PostGIS has been developed by Refractions Research Inc as a research project in open source spatial database technology. PostGIS is released under the GNU General Public License. We intend to continue development as time and resources permit. Our list of future projects includes enhanced technology for data loading and dumping, user interface tools for direct data access and manipulation, and support for advanced topologies at the server side, such as coverages, networks, and surfaces.
The PostGIS website is: http://www.postgis.org/