LinuxLookup.com: Crontab Scheduling Service Guide

"Certain processes must be run at specific times, over and over again. An example of this might be a backup process that is launched each night as the rest of the world sleeps, or a log analyzer that must ran every minute. These processes are ran at certain times or on certain days; the rest of the time, they are not running because they require user interaction (command) or "direction" for lack of a better term. This is where cron comes in handy. It allows you to schedule a specific task to be ran when ever you please, on the minute, hourly, daily, monthly and thus giving you a sense of relief knowing tasks are being executed without you having to baby-sit your system."

"Cron is started from either rc or rc.local and returns immediately, so there is no need to background this command. Cron searches /etc/crontab for system entries and also /var/spool/cron for entries that match users in the /etc/passwd file; found entries are loaded into memory. This occurs every minute as cron "wakes-up" and does several things:

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