Undocumented keywords in /etc/crontab?

[taj cyber-wizard com]

[NB: If this has appeard before, it's because I was having some mailer
problems earlier.]

In the /etc/crontab, the execution timefield is followed by "root 
run-parts" and then a directory. I couldn't find any documentation for the 
"root" or "run-parts" keywords in a crontab. Neither is executable.  Any
ideas what these keywords are doing?

Also, the excution of all files in a directory doesn't seem to be
well-documented. Any pointers on where I can find out more about this custom
usage?

Here are the unmodified contents of the file:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

-- 
Todd A. Jacobs
Network Systems Engineer