A CRON Job is a script that web servers execute at any given time. It is a time-based scheduler for Linux-based operating systems. If your web server uses CRON jobs then you are in luck. Its because CRON Jobs can prevent unnecessary file changes in your system, prevents breakage to your website should you happen to install a faulty plugin, or even back up your entire WordPress blog even when you are sleeping.
CRON Jobs is a nifty tool to prevent problems from occurring and give you piece of mind. However, it is not as easy as you might think it is. Have a look at a detailed explanation of CRON jobs here: http://wphacks.com/wordpress-cron-jobs/