Graphic
You only need to know 4 terms:
- Display manager
- Window Manager
- Graphical User Interfaces (GUI)
- Desktop Environments
Display manager
Examples are LightDM, GDM, KDM, and LXDM. They normally have display manager somewhere in their names. These start the X server at boot and provide a login screen. They often let you select a window manager and/or desktop environment as part of logging in.
Window Manager
Compiz, Metacity, Mutter, W9dk, fluxbox are window managers. There are too many to list. If you want you can check all the packages that provide x-window-manager to get an incomplete list of the window managers in Ubuntu. These only are responsible of drawing the application borders, managing the position of the windows, themes and decorations.
Grapical User Interfaces (GUI)
Anything that the user interacts with in any graphical way, with icons and representations. Window Managers, Display Manager and Graphical shells, applications, etc. falls in this category. Whatever the user is using that is represented predominantly in a graphical way is a GUI.
Desktop Environments
In the Linux world, it's referred to as a set of applications, packages, services, etc. that provides a complete and balanced ecosystem for the user. Unlike Window Manager or Desktop Manager, Desktop Environments take care of everything. From the login screen through your mail application, the network manager, text editor, the system settings, image viewer, file manager, etc. There are 4 well known Desktop Environments in Linux and Ubuntu:
- GNOME
- KDE Desktop Environment
- LXDE: Lightweight X11 Desktop Environment
- XFCE Desktop Environment
These all have a complete set of applications, settings, services, desktop/file/window managers, internet browser, etc.
A full discussion refers to What-kinds-of-desktop-environments-and-shells-are-available
VNC
A VNC server is a program that shares a desktop with other computers over the Internet. You will need a VNC server if you want other people to see your desktop. Every VNC server has different strengths and weaknesses and is appropriate for different uses. This page will discuss each of the VNC servers available in Ubuntu, and ways to configure them for most common uses of VNC.
Server choices
tightvnc offers better compression and performance options. x11vnc offers better security focus ( can be secured with SSH or SSL.) but it's known to be slower. Never used vnc4server or heard much about it, but it's probability not the most popular for a reason...
vnc4server
vncserver -geometry widhtxheight
Modify ~/.vnc/xstartup to start window manager, etc