There seem to be a couple of introductions at these links (note: i haven't checked these out...)
http://www.unixgeeks.org/security/newbie/unix/cron-1.html
http://www.linuxhelp.net/guides/cron/
http://www.linux.com/article.pl?sid=04/04/04/1544250
http://weather.ou.edu/~billston/crontab/
http://www.adminschoice.com/docs/crontab.htm
Also; take a look at the following article from our FAQ section.
That and the man pages should get you going 
Cheers
ZB