Apache: The Definitive Guide, 3rd Edition

ErrorLog

The ErrorLog directive sets the name of the file to which the server will log any errors it encounters.

ErrorLog filename|syslog[:facility] 
Default: ErrorLog logs/error_log
Server config, virtual host

If the filename does not begin with a slash (/), it is assumed to be relative to the server root.

If the filename begins with a pipe (|), it is assumed to be a command to spawn a file to handle the error log.

Apache 1.3 and above: using syslog instead of a filename enables logging via syslogd(8) if the system supports it. The default is to use syslog facility local7, but you can override this by using the syslog: facility syntax, where facility can be one of the names usually documented in syslog(1).

Your security could be compromised if the directory where log files are stored is writable by anyone other than the user who starts the server.