If you are using Apache to host multiple sites with virtual hosting or vhosts, then by default the log output will not show which sites each log entry refers to. To change this edit the following in /etc/httpd/conf/httpd.conf:
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
to read:
LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %v" combined