Yum is installed with centos. This is a short summary for yum.

1.  Here is a yum command summary: 

   yum list - List all available packages.
   yum list installed - List installed packages.
   
   yum check-update
   yum list updates - See if there are updated packages available.
   
   yum update  - Update all installed packages that have a newer version
   available.
   
   yum install   - Install specific packages and their dependencies.
   yum search   - Search all known packages entries (descriptions etc) for word.
   
   yum info   - Show basic information about a package.
   
   yum clean packages  - Eliminate any cached packages from the system.

1. Adding other repositories

   Run this command after startup to obtain the GPG key of the centos mirrors:

		# rpm --import http://mirror.centos.org/centos/RMP-GPG-KEY-centos4