Cron is a timer that periodically executes functions requested by Drupal or its modules. Till a site goes live, you can use a module named Poormanscron. Some modules can't coexist with Poormanscron, like FeedAPI. In that case you have to activate cron through the cPanel of your hosting account.

Steps:

• In the Advanced area of the cPanel click on Cron jobs.
• Choose your experience level Standard.
• Create a new entry by filling out the last area that should be empty.
• In Command to run type:
• /ramdisk/bin/php5 -q /home/youraccountname/public_html/yoursite.com/cron.php (if that doesn't work try /ramdisk/bin/php5 /home/youraccountname/public_html/yoursite.com/cron.php). Lately found this code (put by SimpleScripts??): /ramdisk/bin/php5 -f /cron.php >> /dev/null 2>&1
• Set how often cron should run. I normally run it every hour, but I make sure they crons don't run contemporarilly (a five minute time shift should be okay).
• Email address where cron output should be sent, leave as it is. It should mention your account name.
• Click Save Crontab at the bottom.

Lately the code above didn't work in some sub folders. Solved by adding the script as mentioned in http://drupal.org/node/246091#comment-1161351 under the name cronall.php in one of the working folders and pointing one of the working cronjobs as mentioned above to it.