A geographic information system (GIS) is a database that has latitude and longitude as fields.  It allows objects in the database to be located on the face of the earth.  The system usually includes programs for generating and manipulating maps.  GIS program also have sophisticated programs for analyzing spatial relationships between various objects in the database.

You can learn more at the following web sites:

United States Census

ESRI - Arcview and ArcGIS

Learn about GIS

GIS and public health

National center for health statistics

More links