• edit per-user with crontab -e

* * * * * command

1. min (0 - 59)
2. hour (0 - 23)
3. day of month (1 - 31)
4. month (1 - 12)
5. day of week (0 - 6) (Sunday=0)

• Values can be seperated by commas: eg. 0,20,40
• Values can be divided: eg. */3 (3 times an hour)

Note: The output will be mailed to you, except you do >/dev/null 2>&1

More info: http://www.google.com/search?q=crontab