Its fairly simple actually:
Each user (inc root) has their own crontab (optionally; if they create one).
The 'system' also has a separate one (the ones we said earlier ie /etc/crontab and crontabs called from there).
When you edit your personal crontab, always use
as this create a temp file for you to edit, then when you exit the editor, it copies it to the correct location eg /var/spool/cron/crontabs/<yourname> AND notifies the cron daemon to re-read that file.
http://adminschoice.com/crontab-quick-reference