Good intro:
http://kvz.io/blog/2007/07/29/schedule-tasks-on-linux-using-crontab/
Find crontab at /var/spool/cron/, where you can specify commands to execute at some time interval, for example:
0,20,40 * * * * /usr/sbin/myexecutable > /dev/null 2>&1
The asterisks are time parameters. Minutes, hours, month days, weeks, week days. Multiple entries are separated by commas.