Ubuntu Gnome

[Gunawan]

Hi all,
I believe that ubuntu using gnome.
I have a few questions to ask.
1. I think gnome binary/program save on /usr/bin folder, is that correct?
2. Can I make ubuntu boot to textmode or command line rather than to 
graphical user interface?
3. Can I call gnome from the command line where I think gnome is just a 
program, desktop environment program?
4. In windows, Windows 3.x family we can call windows by issue C:\>win, Can 
I make ubuntu like this?
    go to desktop environment in need?
5. I would like to try install xfce on my ubuntu just for study, can I do 
that?
6. If I extract xfce tar ball on /tmp folder, can I delete them after 
instalation?
7. What is XServer and what XServer do?

Thank you.
I am new to Linux and Ubuntu and just try to study them.
A lot of think to learn :)
Regards,
Gun