There are many desktop environments in linux. Most prominent would be:
KDE
Trinity - KDE fork (response to KDE 4.0 dissatisfaction)
Gnome
MATE - Gnome fork (response to Gnome 3.0 dissatisfaction)
Cinnamon - Gnome fork (response to Gnome 3.0 dissatisfaction)
Unity - Ubuntu's desktop environment (built on top of Gnome 3.0)
XFCE
LXDE
There are many more. Also, there are window managers that have comparable functionality (ex. Enlightenment)
Xfce is a GTK+ based desktop environment for Linux. It is similar in many respects to GNOME, but is designed to use less RAM.
It's a Linux desktop environment. Check out http://www.linux.com/feature/129080
The courses available for Linux include Linux server, Linux desktop, bash shell, and many others.
there are too many Gnome, unity, kde, ....
Slackware Linux has a number of advanced features. Some of them are GCC 4.1.2, XII 7.2.0, HAL, Linux 2.6.21.5, Xfce 4.4.1, The K Desktop Environment and Apache 2.2.4.
Access to reviews of Desktop Linux can be found at Desktop Linux Reviews and Extreme Tech online on their websites. You can also access reviews in YouTube videos and in popular online and paper magazines such as Linux Magazine.
Not all version of Redhat Linux is free but you can download many desktop versions free.
That entirely depends on what desktop environment you are using. At minimum a DE will let you change your wallpaper, icons, and how many workspaces you have.Plasma Workspaces (KDE's desktop environment.) offers you the ability to have different wallpapers per workspace and desktop widgets. On top of this Plasma Workspaces offers another feature called "activities" which is similar to workspaces.
A .desktop file is a plaintext configuration file. What it does is basically tell the desktop environment to display an icon and to launch a program when it is clicked on. It is the Windows equivalent of a shortcut. They can be created are manually by a user, or by the installer for a program.
It's about as easy to use as any Linux distribution. The main point it lacks in is hardware support, which isn't quite as good as Linux's. It uses the KDE desktop, which is also found in many Linux distributions.
Yes it is possible to do so. Once you have them, at the login screen you should find an option that allows you to choose which desktop environment you want to use.
There is no standard reader for manpages for a graphical environment. You can either use the man command in a terminal emulator, or a graphical program like Tkman or TkViewMan.