Ubuntu by default uses the Gnome user interface. Fluxbox is an alternative window manager for Ubuntu that is extremely lightweight and customizable.

Installing fluxbox is really quite simple. First you will want to make sure that you've enabled the universe repository.

To install Fluxbox, open up a terminal window and type in the following command:

sudo apt-get install fluxbox

You will be prompted for your password, and the installation will continue. Once you are done installing, you will need to exit XWindows by using Ctrl+Alt+Backspace or just rebooting the computer.

At the Ubuntu Login prompt, click the Options button in the bottom left hand corner:

You will be prompted to choose from a list of Window Managers that are installed on your system. Select Fluxbox and continue to login.

Fluxbox is really simple. The main menu can be seen by right-clicking on the desktop:

At the bottom of the screen, you will see a taskbar that shows you the running programs and lets you switch between workspaces:

The Fluxbox window style is a lot different than you may be used to. Here's a screenshot for your enjoyment:

Fluxbox is definitely not for everybody, but it's about as lightweight as you can get, so it's probably useful for low memory systems.