This 'plugin' enables WP to use databases supported by PHP's PDO abstraction layer. Currently, mysql and sqlite drivers are provided.
WordPress has for a long time, and for good reasons, been locked into using mysql as its database storage engine. There is a good discussion of 'why' in the codex
But this design choice has ramifications; not least because mysql's implementation of sql is not standard. Even with the use of the EZSQL abstraction layer bundled with WordPress, this makes plugging in other databases very difficult.
PDO For WordPress is a step towards eliminating this difficulty. Think about this 'plugin' in four steps:
See below/other notes for details of known limitations