PHP-FPM is “deciphered” as “PHP FastCGI Process Manager” and is a patch for php to greatly improve FastCGI SAPI usage in production. It adds a bunch of additional features to php’s fastcgi such as: easy php-process daemonization (with ability to specify uid/gid/chroot/log-file), safe php-processes restart (without losing requests), custom error-handling and accelerated file-upload support (requires additional support from web-server).

There’s not much documentation in english, currently, but, again, there is a good chance that it will be added really soon.