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).
L. Stenneth, O. Wolfson, P. Yu, und B. Xu. Proceedings of the 19th ACM SIGSPATIAL International Conference on Advances in Geographic Information Systems, Seite 54--63. New York, NY, USA, ACM, (2011)