You may be asking, "What is PHP-FPM, and why should I care?" PHP-FPM is a patch for PHP core that handles the starting, stopping, and restarting of FastCGI processes as needed. This is important because nginx can only interface with PHP via FastCGI, unlike Apache, which loads the whole PHP environment right into itself.
Right now, the only downside to setting up the FastCGI interface with nginx takes a bit more work than the normal mod_php Apache handles so well. The PHP-FPM patch helps with some of this by making the FastCGI support directly embedded into PHP, removing the need for a lot of external setup.