Try these two links:
- Ubuntu Help. Describes the use of cron and the /etc/cron.daily, /etc/cron.hourly etc. directories used by root.
- Linux Forums. Goes into more detail about crontabs for individual users.
What exactly don't you understand? The trick with cron is that you should always use full path names or else scripts / programs may not be run and cron will not appear to work.