Syslog is an operating system administration logging tool, where messages are routed by facility and severity. It is more suitable for medium to large sites, and would not be suitable for shared hosting environments. In the file /etc/syslog.conf you define where messages go for any combination of facility and severity.

For more information on syslog facilities, severity levels, and how to setup a syslog.conf file, see UNIX/Linux syslog.conf and PHP's syslog.