Crontab is a mechanism for launching tasks in the background. Great for automatic backups, system heartbeat, and other periodic activities. There are two ways to launch crontab tasks: System-wide, User-specific.

User-specific - items listed in the special file shown using crontab -l will be run as though invoked by the specific user. Typically user-specific crontab files are stored in /var/spool/cron and should only be changed using crontab -e.

$ crontab -l
  00 * * * * /usr/bin/php /var/www/html/foobar.com/community/mail_digests.php

$ crontab -e
  ...

System-wide - For system wide tasks a master crontab is typically invoked by root user once a minute. See /etc/crontab for details (don't even THINK about editing this file). The cron.d folder is equivlent to running crontab -l as root. Typically services (i.e. installable packages) that use cron will use the system-wide form. For manually based backups and other highly custom/non-service/administration tasks the user-specific form is a better choice - mostly because these tend to need frequent adjustments and using crontab -e is cleaner than mucking about in /etc/cron... directly.

# cat /etc/crontab

  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

# ls -ld /etc/cron.*

  drwxr-xr-x  2 root root 4096 Feb 17  2004 cron.d
  drwxr-xr-x  2 root root 4096 Mar 22  2005 cron.daily
  drwxr-xr-x  2 root root 4096 Feb 15  2004 cron.hourly
  drwxr-xr-x  2 root root 4096 Mar 21  2005 cron.monthly
  drwxr-xr-x  2 root root 4096 Mar 21  2005 cron.weekly

# ls -lr cron.*

  cron.weekly:
  -rwxr-xr-x  1 root root 414 Oct 13  2003 makewhatis.cron
  -rwxr-xr-x  1 root root 277 Feb 15  2004 0anacron
  -rwxr-xr-x  1 root root 414 Nov 18  2004 00-makewhatis.cron

  cron.monthly:
  -rwxr-xr-x  1 root root 278 Feb 15  2004 0anacron

  cron.hourly:

  cron.daily:
  -rwxr-xr-x  1 root root  136 May 11  2004 yum.cron
  -rwxr-xr-x  1 root root  193 Feb 15  2004 tmpwatch
  -rwxr-xr-x  1 root root   82 Apr 16  2004 slocate.cron
  -rwxr-xr-x  1 root root  104 Dec 28  2004 rpm
  -rwxr-xr-x  1 root root 1603 May  5  2004 prelink
  -rwxr-xr-x  1 root root  418 Oct 13  2003 makewhatis.cron
  -rwxr-xr-x  1 root root  180 Feb 15  2004 logrotate
  -rwxr-xr-x  1 root root  276 Feb 15  2004 0anacron
  -rwxr-xr-x  1 root root  135 Mar 27  2004 00webalizer
  -rwxr-xr-x  1 root root  418 Nov 18  2004 00-makewhatis.cron
  lrwxrwxrwx  1 root root   28 Mar 22  2005 00-logwatch -> ../log.d/scripts/logwatch.pl

  cron.d:
  -rw-r--r--  1 root root 46 Aug  7 20:23 www-back

# cron.daily/rpm 

  #!/bin/sh

  rpm -qa --qf '%{name}-%{version}-%{release}.%{arch}.rpm\n' 2>&1 \
      | sort > /var/log/rpmpkgs

# cat /etc/cron.d/www-back

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

  # 10:05 PM daily, backup www
  05   22 * * *   root sh /public/archive/www-back.sh | mail -s "www backup" backups@foobar.com

Installing with yum ...

# yum list \*cron\*

  Installed Packages
  crontabs.noarch       1.10-8                  installed
  Available Packages
  anacron.i386          2.3-45.el5.centos       base     
  vixie-cron.i386       4:4.1-77.el5_4.1        base     
  yum-cron.noarch       0.6-1.el5.centos        extras   

# yum install vixie-cron

  ...
  Installed:
  vixie-cron.i386 4:4.1-77.el5_4.1                                              

# crontab -l

  no crontab for root 

# service crond status

  crond is stopped

# service crond start

  Starting crond: [  OK  ]  

Don't forget to start crond after installing it.  Typically crond sends script output as email to user accounts.  You may need to check that sendmail (or equivelent) and mailx are installed and running for user accounts to recieve email.