Abstraction layer

My description might not be exact right however I believer what I need is to get used to the concept. Things do not need to exact.

I understand the abstraction layer as below.

http://en.wikipedia.org/wiki/Abstraction_layer

KDE
Shell
Kernel
Firmware Bios
Hardware HDD, RAM

KDE: Desk top manager. Except this desk top manager Gnome is also well known desk top manager.

Shell: a shell is a piece of software that provides an interface for users. Typically, the term refers to an operating system shell which provides access to the services of a kernel. There are two kinds of shells.

http://en.wikipedia.org/wiki/Shell_(computing)#Text_.28CLI.29_shells

GUI (graphical user interface)
One of the well known shells is “the X windowing system”. This software give us more friendly interface. It is not related to something Microsoft windows at all. It is a GUL got Linux, Windows.

Another famous GUI is ”windows explore”. It is not related to “Internet explore” at all.
http://en.wikipedia.org/wiki/Windows_Explorer

CLI(command line interface)
I often type bash command in the black console window. This sort tof unfriendly interface is supposed to be CLI.
We have many different commands type: CMD, BASH, CSH, KSH, BSH.
Among these command, bash is the most popular and I have used before.

*************