i just installed RedHat 9.0, and find there are several kinds of shells (i guess so), like screen session, linux console, midnight commander, shell, root console, root midnight commander, what r the differneces among them

Actually, screen and midnight commander are both programs that you run in a shell console.

If you've used Windows, you might think of the 'shell' as the 'command prompt'. 'root' is equivalent to 'administrator' in Windows; root essentially has unlimited privileges to read, write or execute any file on your computer.

Midnight commander is a filemanager similar to Norton Commander from the DOS days. It's very efficient; I prefer it over the graphical filemanagers like Konqueror or Nautilus. Try running it; it has self-explanatory menu options conveniently displayed. Tip: use the INSert key to select multiple files for copying, moving, deleting, etc.

Screen is also an awesome program. It's a like a windowmanager for your console. I'm using it right now in an ssh session from my laptop to my server; when I start a job on the server, I can detach the screen, and turn off my laptop but the job will continue running on the server. There's a better explanation of screen over at kuro5hin.org.

before we had the GUI the shell was the thing people liked to configure. Different people thought that different shells worked better than others like the "cshell" and "bourne again shell".

We now have GUIs with window managers like KDE, Gnome, Window Maker etc. The difference between them! nothing really they just provide different means of doing the same stuff.