Install desktop environment (GUI) on Ubuntu Server 12.04

You know after you install ubuntu server it won't have graphical user interface. Most of the time you won't need one, why you installed it in the first place. To make system as light as possible to run only essential services like web-server, database-server, etc. But sometimes you might change your mind and you server application with graphical user interface.
You have quite many options to choose for graphical desktop.

Installing KDE-based desktop options:

1st update your system:

$ sudo apt-get update

In case you want to install packages released for newer version of ubuntu then you might wanna add baclport repositories. But this not advised and my render your system unstable.

Adding backport repo:

$ sudo add-apt-repository ppa:kubuntu-ppa/backports

Now we have several options

$ sudo apt-get install kubuntu-desktop
or
$ sudo apt-get install kde-standard
or
$ sudo apt-get install kde-full
or
$ sudo apt-get install plasma-desktop
or
$ sudo apt-get install kde-plasma-desktop

kde-plasma-desktop,plasma-desktop are more lightweight compared to full versions, of course.