Due to the tight integration into PHP 5.3 (and later), mysqlnd eliminates the dependency on the MySQL client programming support when the database extension(s) and the database driver are built with the support for mysqlnd. mysqlnd is not another PHP extension like mysqli nor it has an exposed API to the userland. It is a library that provides almost similar functionality as that of the MySQL client library, libmysql. mysqlnd and libmysql libraries implement the MySQL communication protocol - hence both of those libraries can be used to connect to the MySQL Server.
The Sun blog entry also walks you through the installation and configuration of a PHP instance with the native driver installed (with the upcoming PHP version, 5.3).