DE or Desktop Environments are terms used in relation to Linux. DE’s are simply Graphical User Interface (GUI). GUI assist the user in using the computer without the knowledge of commands or any programming language. Linux was previously considered as a operating system which is only used by the Geeks, and that’s because so many commands were used to do a particular task. However this has changed over time, and now, even a person who doesn’t have any technical/programming knowledge can use Linux. Thanks to its GUI.
A desktop environment typically consists of windows, gadgets, folders, icons, wallpapers, screensavers and toolbars. Desktop Environment is a way for the user to interact with the computer and make the task easy for him.
Some famous Desktop Environments are KDE, GNOME, Xfce, CDE, etc. So before downloading and trying out any Linux distro just first see what DE it has and whether you are comfortable with it or not.
My favorites are KDE and GNOME. KDE stands for K Desktop Environment. And GNOME stands for GNU Object Model Environment.
The two most popular Linux distros are Ubuntu and openSUSE. If you are installing Ubuntu then you’ll get GNOME Desktop Environment by default. And if you are installing openSUSE then you’ll get KDE by default.
There are many flavors of Linux, so keep on trying different flavors and this way you’ll know which one suits you.
Also checkout this Desktop Environment Wiki.