HOWTO Use the /etc/cron.* infrastructure on Debian

This article is incomplete and was first written in June 2006
for the BeezNest technical website (http://glasnost.beeznest.org/articles/339).
On Debian, an infrastructure is in place in directories /etc/cron.* to extend and improve the crontabs.

Why ?

Usually, crontabs are managed at the user level, which can start crontab -e to edit its own or crontab -l to just list it.