Crontab concept

Hi,

I try to understand how is managing the "execution" of /etc/crontab which configure the launch of the scripts stored in the folder /etc/cron.hourly ... and so on. No crontab job are define (I mean in the Unix point of view, the cron job you can identify with the command crontab -l). I saw, looking at the script /usr/bin/run-parts a comment : "concept taken from Debian". I'm familair with UNIX AIX or SOLARIS but a bit lost with Debian ... Does anyone can explaine me this concept or where i can find a Howto that explain this.

Thanks,
benja