Webmin is a web based administration tool to do daily admin activities.
by using this tool we can configure.
1. User management
2. Disk management such as RAID, LVM etc.
3. Disk Quotas
4. Services/Servers such as apache, DNS, PHP, Mysql etc
5. Will allow us to configure/control many systems through single interface(ie through a web browser).

For example, if you want to control your Linux machine remotely through machines such as windows or a mobile device. We can do the system admin activity through a web broswer. Some time I used to login to my linux server which is located in SJ, USA through my Sony Ericsson C702 mobile phone to take backups.

Installation Webmin in Ubuntu

If you like to install and update Webmin via APT, edit the /etc/apt/sources.list file on your system and add the line :

deb http://download.webmin.com/download/repository sarge contrib

You should also fetch and install a GPG key with which the repository is signed, with following commands. 

cd /root
wget http://www.webmin.com/jcameron-key.asc
apt-key add jcameron-key.asc


You will now be able to install with the commands :

apt-get update
apt-get install webmin

Installation in Redhat/Fedora

yum install webmin

After installation webmin server will work on localhost at 10000 port, which is default port for webmin.

To access webmin through your browser

http://localhost:10000

Why a unix admin should know about Webmin?
Ans :
Webmin is a one stop to administer your machine we can manage allmost all the servers, disk management, user management etc. Which is supported on different operating systems, recently it is supporting Windows too(with limitations) .

Related concept:   8 Ways To Mount SMBfs (SAMBA FILE SYSTEM) In Linux.

Which are other operating systems we can install webmin to administer the system?
Ans :
 It is supported in many unix and Linux flavours such as
1. Redhat, Fedora, SuSE and Mandrake through RPM packages
2. Debian and Ubuntu through .deb packages
3. HPUX, FreeBSD and Slackware Linux through source code(eg: .tar.gz and .tar.bz)

4. MS Windows through exe.