What is cron?
  cron is the name of a UNIX program enabling users to run periodic processes
  at predefined times. The cron daemon (crond) is normally started with system
  initialisation, crond consults the crontab configuration files every minute.
  cron has a systemwide configuration file named crontab while individual
  users may create their own cron configuration file using the crontab command.  
  System examples of cron periodic processes would be to fetch email from a
  server every x minutes or rotate the system logs once a week. A users use of
  cron may include a regular backup of the home directory.

Why the name cron?
  cron is simply derived from the greek word cronos meaning time.
  "vixie cron" being modified and enhanced for linux was developed by Paul
  Vixie and is referred to in this document simply as cron.
  
System wide services are run from the following directories at specific preset 
times:
  /etc/cron.hourly scripts run every hour at one minute past the hour
  /etc/cron.daily scripts run every day at 04:02am
  /etc/cron.weekly scripts run weekly on Sunday at 04:22am
  /etc/cron.monthly scripts run on the first day of each month at 04:42am

Below is a listing of the /etc/cron.daily directory on a Red Hat 9 system:

total 24
lrwxrwxrwx    1 root     root           28 Jan 21  2006 00-logwatch ->
../log.d/scripts/logwatch.pl
-rwxr-xr-x    1 root     root          276 Jan 25  2003 0anacron
-rwxr-xr-x    1 root     root           51 Jan 25  2003 logrotate
-rwxr-xr-x    1 root     root          418 Feb 11  2003 makewhatis.cron
-rwxr-xr-x    1 root     root          104 Feb 28  2003 rpm
-rwxr-xr-x    1 root     root          132 Feb 20  2003 slocate.cron
-rwxr-xr-x    1 root     root          193 Feb 11  2003 tmpwatch

Listing descriptions and fuctionality:   

- The logwatch symbolic link connects to script logwatch.pl a system log
  analyser and reporter.
  
- 0anacron is a executable script for updating anacrons time stamps. anacron
  catches any cron jobs that were missed when the system was turned off.

- logrotate, also a script, used to rotate system log files.

- The makewhatis.cron script creates the database for man -f or whatis
  command line (keyword) searches.

- rpm script creates a database of rpm pakages installed on the system.

- slocate.cron is a script for creating a database providing a secure way to 
  index and quickly search for files on your system.

- tmpwatch script automatically removes files over a certain age. Files in the
  /tmp directory are removed if more than ten days old and 
  files in the /var/tmp directory are removed when older than 30 days.

The crontab file located in /etc directory, /etc/crontab, is a systemwide 
configuration file for the cron daemon. An example is given here:

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

# run-parts command is used to run all executable scripts in a directory
# min  hr dom mon dow 	user   script    command
  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
  49  22   *   *   * 	root             /sbin/shutdown -h now
#
#min=minute, hr=hour, dom=day of month, mon=month, dow=day of week

Note how the start time of each script is determined:
- An asterisk functions like a wild card when placed in a field. Thus an
  asterisk in the min column means every miunte, the hr hour column means 
  every hour. 
- min (range is from 0 - 60) at which this job should be run.
    To specify multiple values use commas in a list. e.g. 0,10,12,34.
    To run a job every minute in a range, specify like this 5-10, 24-36.
- hr (range 0 - 24) accomodating lists, ranges as for min
- dom (range 1 - 31) accomodating lists and ranges as for min
- mon (range 1 -12) accomodating lists and ranges as for min
- dow (range 0 - 7 where 0 and 7 are both Sunday) lists and ranges are
    accomodated also
- The last entry in the systemwdie crontab file is executed at 22:49pm each
  day shuting down the computer automatically.
 
crontab    (1) maintain crontab files for individual users
cron       (8) cron daemon to ececute scheduled commands
anacron    (8) runs commands periodically with a frequency specified in days.
               does not assume machine is running 24hrs a day and can be used
               to control daily, weekly and monthly jobs that are usually
	       controlled by cron
anacrontab (8) configuration file for anacron