Ubuntu comes in 3 main sizes, Large, Small, and tiny to meet different needs.

Complete description of each version after the break.

Ubuntu Linux Desktop comes in a variety of flavors. There are 5 major flavors in all. The Ubuntu desktop family is made up of:
Ubuntu 7.10

Ubuntu is a community developed, linux-based operating system that is perfect for laptops, desktops and servers. It contains all the applications you need - a web browser, presentation, document and spreadsheet software, instant messaging and much more.

Edubuntu 7.10

Edubuntu is an education version of Ubuntu, designed to allow a teacher or network administrator to setup and maintain a classroom quickly and easily. It includes educational applications, an office suite, web browser, communication tools, and a secure operating system.

Kubuntu 7.10

Kubuntu is a user friendly operating system based on Ubuntu and KDE, the K Desktop Environment. Kubuntu 7.10 comes with the KDE 4 Technical Preview which is a great opportunity for KDE enthusiasts to view and contribute to the future of KDE.

Gobuntu 7.10

Gobuntu 7.10 is the debut full release of the Ubuntu version that is entirely composed of open source non-restricted software. For a certain user type, Gobuntu will act as the test bed for developing a user-friendly operating system with no compromise in terms of the open source philosophy.

Xubuntu 7.10

Xubuntu is built on the Ubuntu base and is ideal for old computers or or any system which does not meet the minimum requirements for Ubuntu. Xubuntu 7.10 adds many of the GNOME features currently in the full Ubuntu release making the Xubuntu release a more feature full experience for users while still working on less powerful machines.

Source: Ubuntu Family Announcement

Now, What happens when you want to run a server, you don’t need KDE, or even X. Just a nice little command line interface. Ubuntu has you covered. There are currently 2 versions of Ubuntu server available for download 6.06 and 7.10. Both of these are minimal but complete (depending on your needs and options you check during install). For more information about these visit Ubuntu Server Edition

But.. what happens if we “squeeze” just the juice of life out of the server edition, and savor ever drop into a truly minimal server OS. You get Ubuntu JeOS (pronounced Juice). This is a version is designed for only 1 type of hardware, Virtual hardware. The 7.10 JeOS distribution has been refined and tested to run on VMware Server and ESX Server only. The next version 8.04 will be a little more friendly to other virtualization platforms including virtual box and virtual iron.

Now what can you do with JeOS? They are designed to be the starting point to virtual appliances. If you want to build a LAMP appliance, or a Java Appliance you can use JeOS as the base because it is smaller and optimized to run on VMware Products. Smaller image, less overhead, fewer update issues it is a true win win for everyone.

If you are looking for a complete step by step directions on building your first Virtual Appliance follow the directions here “How to Develop Virtual Appliances Using Ubuntu JeOS”

For a complete overview of the JeOS ideals and system check out this Get Juiced! - The Console

Also for those on the other side of the fence, Windows 2008 has have a “JeOS” option, where you can install Windows 2008 without the gui.
Windows Server 2008 Product Overview

The new Server Core installation option of Windows Server 2008 allows for installation of server roles with only the necessary components and subsystems without a graphical user interface. Fewer roles and features means minimizing disk and service footprints while reducing attack surfaces. It also enables your IT staff to specialize according to the server roles they need to support.

And for you die hards there is always “Damn Small Linux” available from http://damnsmalllinux.org/

DSL was originally developed as an experiment to see how many usable desktop applications can fit inside a 50MB live CD. It was at first just a personal tool/toy. But over time Damn Small Linux grew into a community project with hundreds of development hours put into refinements including a fully automated remote and local application installation system and a very versatile backup and restore system which may be used with any writable media including a hard drive, a floppy drive, or a USB device.