PostGIS is a set of extensions to the relational database management system PostgreSQL, which provide access to spatial constructs, operators, and functions. In other words, PostGIS is a spatially-aware version of Postgresql. Paul Ramsey of Refractions Research has written up a nice summary of how were are using PostGIS.

 
Tuning Tips:

1. Adjust kernel resources to allow larger memory allocation: (documentation)
2. Increase the shared_buffers to a larger number (16000)
3. See attached PDF at the bottom of this page for excellent ideas from Kevin Neufeld

 
PostGIS Syntax Examples:

1. PostGIS Spatial SQL cheat-sheet
2. Official Documentation
3. PostGIS Wiki

 
PostGIS Presentations:

1. Tips for the PostGIS Power User (Kevin Neufeld, Refractions Research)
2. PostGIS Spatial Database: Introduction and Case Studies (Paul Ramsey)
3. Transitioning Low Earth Orbit Satellite Archive Data from Informix (Geodetic DataBlade) to PostgreSQL (PostGIS) (Churngwei Chu, NASA/SSAI)
4. Intro to PostgreSQL/PostGIS and MapServer (Arnulf Christ)