I guess the OP is referring to the directories e.g. SuSE has, like /etc/cron.daily, /etc/cron.hourly ... I never found any documentation concerning this.
<edit> It is described in the LSB (Linux Standard Base) documents or rather in the links from there to file system hierarchy / root file system / cron. But not how it is really used in easily understandable form. Ah, well... </edit>
For the normal
use of cron you have to use crontab -e, since the resulting file holds more information than you can see in your editor.
If you have a workstation which is not
running around the clock or want extended capabilities have a look at fcron (http://fcron.free.fr/