We able to download the packages by YUM command without any installing it. There are two ways,

1. downloadonly plugin

2. yumdownload utility

First,  check downloadonly plugin how it's working ?

Install that packages using yum command,

RHEL6

# yum install yum-plugin-downloadonly

RHEL5
 
# yum install yum-downloadonly

Install the downloadonly plugin
 
# yum install yum-plugin-downloadonly

Dependencies Resolved

================================================================
 Package                                   Arch           Version                Repository       Size
================================================================
Installing:
 yum-plugin-downloadonly      noarch      1.1.30-17.el6_5        updates          21 k

Transaction Summary
================================================================
Install       1 Package(s)

Installed:
  yum-plugin-downloadonly.noarch 0:1.1.30-17.el6_5                                                                                                                                              

Complete!

Note :

Possible to download multiple packages on the same command

Unfortunately the plugin doesn't support to "yum goupinstall"

Before utilizing the plugin, be assured "enabled=1" in /etc/yum/pluginconf.d/downloadonly.conf like,
#vi /etc/yum/pluginconf.d/downloadonly.conf
[main]
enabled=1

To determine packages within a specific group "yum groupinfo"

You can download specific packages with name and version(like, nagios-3.5.1-1.el6.i686.rpm)

If you will not use the --downloaddir option, by default files are stored in /var/cache/yum  in rhel-{arch}-channel/packages

Syntax :
 
yum install --downloadonly --downloaddir=<directory name> <package name>

How to use it?
 
# yum install --downloadonly --downloaddir=/root nagios

...
...
exiting because --downloadonly specified

Second, check Yum downloader how it's working ?

 yumdownloader is used for download RPM packages from Yum repositories

Install the yum-utils package using yum command
 
 # yum install yum-utils
    
=====================================================
 Package           Arch          Version                   Repository           Size
=====================================================
 yum-utils       noarch       1.1.30-17.el6_5        updates              102 k

Transaction Summary
=====================================================

Installed:
  yum-utils.noarch 0:1.1.30-17.el6_5   

Note:

By default, the packets stored in the current working directory

If you will use --destdir option used for saving the files to a specified location,

Syntax
 
 yumdownloader <package name>

How to use it?
 
# yumdownloader kernel
...
kernel-2.6.32-431.17.1.el6.i686.rpm                                   |  26 MB     00:06      | 1.2 MB     00:02     
# ls
kernel-2.6.32-431.17.1.el6.i686.rpm