What do I put for the cron job Command ?

Shared, Reseller, SEO hosting, and other (VPS and Dedicated with Easy Apache 3 installed)

PHP

Command to run a PHP5 cron job:
php /home/user_name123/public_html/cron.php

Optional flag sometimes required for a PHP cron job:
php -q /home/user_name123/public_html/cron.php

Command to run a PHP4 cron job:
/usr/local/php4/bin/php /home/user_name123/public_html/cron.php

All hosting types

More

Command to GET a remote file:
/usr/bin/GET http://www.some-domain.com/file.php

Command to run a CGI cron job:
perl /home/user_name123/public_html/cgi-bin/file.pl

SSH Extras

Command to run a shell script cron job:
/bin/sh /home/user_name123/public_html/file.sh

Command to import a database:
mysql -u mysql_user -ppassword database_name < backup.sql

Command to export a database:
mysqldump -u mysql_user -ppassword database_name > backup.sql

VPS and Dedicated without Easy Apache 3 (unlikely, we install EA3 by default)

PHP

Command to run for a PHP4 cron job:
/usr/bin/php /home/user_name123/public_html/cron.php

Command to run for a PHP5 cron job:
/usr/local/php5/bin/php5 /home/user_name123/public_html/cron.php