FPM (FastCGI Process Manager) is an alternative PHP FastCGI implementation with some additional features (mostly) useful for heavy-loaded sites.
These features include:
advanced process management with graceful stop/start;
ability to start workers with different uid/gid/chroot/environment, listening on different ports and using different php.ini (replaces safe_mode);
stdout and stderr logging;
emergency restart in case of accidental opcode cache destruction;
accelerated upload support;
"slowlog" - logging scripts (not just their names, but their PHP backtraces too, using ptrace and similar things to read remote process' execute_data) that are executed unusually slow;
fastcgi_finish_request() - special function to finish request and flush all data while continuing to do something time-consuming (video converting, stats processing etc.);
dynamic/static child spawning;
basic SAPI status info (similar to Apache mod_status);
php.ini-based config file.