PostgreSQL for WordPress is a special 'plugin' enabling WordPress to be used with a PostgreSQL database.
PostgreSQL for WordPress (PG4WP) gives you the possibility to install and use WordPress with a PostgreSQL database as a backend. It works by replacing calls to MySQL specific functions with generic calls that maps them to another database functions and rewriting SQL queries on the fly when needed.
Currently, support is focused on PostgreSQL, but other databases can be added quite easily by providing the appropriate 'driver'. MySQL driver is enclosed, which just does "nothing". If you need/wish support for another database, please feel free to contact the author, writing a driver is not really hard if you know a bit about SQL and the database you want support for.
If you want to use this plugin, you should be aware of the following : - WordPress with PG4WP is expected to be slower than the original WordPress with MySQL because PG4WP does much SQL rewriting for any page view - Some WordPress plugins should work 'out of the box' but many plugins won't because they would need specific code in PG4WP
You shouldn't expect any plugin specific code to be integrated into PG4WP except for plugins shipped with WordPress itself (such as Akismet). PG4WP 2.0 will have a mechanism to add plugin support.