Yellow dog Updater, Modified (Yum) is the default package manager used in CentOS ( all versions ). It is used to install and update packages from CentOS (and 3rd party) Repositories.
Managing Software with Yum is an online manual, available at http://www.centos.org/docs/4/html/yum/ - this should be applicable to all versions of CentOS, with a few changes.
CentOS 4 also contains the Yum Extender (a GUI front end for yum), documention on this is available at http://www.centos.org/docs/4/html/yumex/
Yum 2.4.x on CentOS 4 now has the capability of running plugins, some of which are hosted online within the CentOS Repositories.
Enabling yum plugins: In order to use any plugins with yum on CentOS 4, you must first edit the file /etc/yum.conf and add the following line:
Once plugins are enabled in yum.conf, you may install yum plugins and use them. CentOS 5 has this option enabled by default.
Some yum plugins are available as seperate rpm packages. Currently in CentOS-4 those are protectbase, priorities and fastestmirror. These plugins are highly recommended for CentOS-4 users who are using the default update method. (Use only priorities OR protectbase ... they can not be used together).
/FastestMirror: yum plugin to determine and use the fastest CentOS mirror (CentOS 4 and 5 )
/ProtectBase: yum plugin that prevents external repositories from overwriting CentOS packages ( CentOS 4 and 5 )
/Priorities: yum plugin that allows assigning priorities from 1 to 99 to yum repositories.
The CentOS-4 only: yum-utils also contains some yum plugins that are not enabled by default. Those plugins are located in the directory /usr/share/doc/yum-utils-0.5/plugins after the package yum-utils is installed.
Please review those plugins and if you would like to install them for use, copy the plugin_name.conf file to /etc/yum/pluginconf.d/ and the plugin_name.py file to /usr/lib/yum-plugins/ to enable that specific plugin.
External URL's that might come in handy:
Yum Home page : http://yum.baseurl.org/
Yum Bug Reports : http://yum.baseurl.org/report
Yum Wiki ( contains an FAQ specific to Yum ) : http://yum.baseurl.org/wiki/Faq
( Note: keep in mind that the yum version used in CentOS might not always be the same as the version reported / used upstream )