Essential reading: man 5 crontab
The first and most important thing to remember about crontabs is that there are two types of crontab. There’s the system crontab, kept in /etc/crontab (and files in /etc/cron.d/, /etc/cron.daily/, /etc/cron.hourly/ and /etc/cron.weekly/). Then there’s the user crontabs, which are edited by running crontab -e and viewed with crontab -l. These have subtly different formats, and if you put the wrong format in the wrong place, things will not work (or at best, not as you expect them to).
System crontab format
Each line in the system crontab contains 7 whitespace-separated [continued…]