Also, which will be most appropriate for my old system?
The main two differences between Ubuntu and its derivatives are the backing and the default installed packages. For all official derivatives, you can convert between variants by installing certain packages.
Here is a small list:
Ubuntu comes with the Unity desktop environment. The underlying Unity platform is still GNOME, but instead of using the GNOME Shell interface, Unity uses the Unity shell.
Ubuntu and Unity are commercially backed and supported by Canonical.
To convert an installation into regular Ubuntu, install ubuntu-desktop .
This is the KDE flavor of the typical Ubuntu (which is GNOME driven). The primary difference is that Kubuntu comes with KDE as the default Desktop Environment, as opposed to GNOME with the Unity shell.
Kubuntu is sponsored by Blue Systems. Canonical stopped backing it in 2012, but it is still an officially recognized Ubuntu variant, which means that it gets build machines, test machines, CD image distribution servers, etc. dedicated to it.
To convert an installation into Kubuntu, install kubuntu-desktop .
Ubuntu GNOME aims to have a mostly pure GNOME desktop installed by default.
Ubuntu GNOME is community-driven. It was originally called Ubuntu GNOME Remix; it was renamed to Ubuntu GNOME when it became officially recognized. Ubuntu GNOME is very new: its first release as an official derivative was 13.04 Raring Ringtail, which as of 6/9/13 is the current release.
To convert an installation into Ubuntu GNOME, install ubuntu-gnome-desktop .
This is a lighter weight, highly efficient and optimized flavor of Ubuntu designed to run on older computers. It uses XFCE which is a proven faster Desktop Environment than both KDE and GNOME. This is a typically simpler slim interface.
Xubuntu is community-driven. It is also officially recognized by Canonical (see Kubuntu for what that means).
To convert an installation into Xubuntu, install xubuntu-desktop .
Ubuntu Server is optimized for use as a server. It does not come with X.Org, and as such does not use any graphical environment like a desktop environment or a window manager. Instead, it comes only with a CLI environment.
Ubuntu Server is commercially backed and supported by Canonical.
The counterpart to Xubuntu - Mythbuntu is designed to be an entertainment powerhouse. Focused around being a Media Center it includes many drivers and setups for TV Tuners, TV Out cards, and has a Media Center application (MythTV) integrated into the Desktop Environment to facilitate the entertainment powerhouse it advertises.
Mythbuntu is an officially recognized Ubuntu variant.
To convert an installation into Mythbuntu, install mythbuntu-desktop .
Lubuntu takes the aims of Xubuntu and pushes the Desktop Environment to an even more bare bones lightweight variant: LXDE (Lightweight X11 Desktop Environment) which is supposed to be an even more efficient, power saving, fast, lightweight Desktop Manager than XFCE.
Lubuntu is community-driven. It is officially recognized as a variant by Canonical.
To convert an installation into Lubuntu, install lubuntu-desktop .
I've used the first three before and I believe all have some backing from Canonical (or other corporate backing) while Lubuntu is still relatively new and community driven. Though if you have a very old/slow computer it certainly might be what you're looking for.
Figured I'd add the other flavors of Ubuntu I knew where out there
In addition to the above listed there is also:
This flavor is designed with Educational intent. Runs very close to the vanilla Ubuntu release though it comes with many additional applications that are best suited for an Educational environment. It also is configured and stylized with kids in mind.
Edubuntu is an officially recognized variant.
This flavor is geared towards those who deal with multimedia (Video, Audio, Graphics, Design) on a daily basis. Comes bundled with many applications, codecs, and drivers required to facilitate those activities.
Ubuntu Studio is an officially recognized variant.
Ubuntu CE (Ubuntu Christian Edition) is Ubuntu designed for Christians who wish to maintain an "Christianly" lifestyle. This comes bundled with a lot of religious software and security tools to help protect moral religious obligations.
Ubuntu CE is an unofficial derivative.
This project is idle, but it's intent was to bring stronger hardened security to the stock Ubuntu installation. It targets security practitioners like penetration testers.
Nubuntu is an unofficial derivative.
Fluxbuntu does not come with a desktop environment like GNOME or KDE. Instead, it only comes with a window manager called Fluxbox.
Fluxbuntu is community-driven. It is an unofficial variant.
Pretty extensive but I will try to give it my best:
Ubuntu - The most common and known one of all. It uses the Unity Desktop as default and creates an standard desktop format for TVs, smartphones, netbooks and desktops. Because of this, at least for now, Unity is not completely customizable. This will change over time while the 5.x and 6.x versions keep on progressing. Unity uses the gnome 3 libraries to work. Ubuntu is not considered a lightweight distro although it works on some low end hardware. As an alternative, Ubuntu let's you install Gnome Shell so you can have the Gnome 3 experience. Both are similar in several aspects but work in some different ways. This is basically a choice option, something that you will find that the Ubuntu community offers.
Kubuntu - Most Windows users love this one since the transition from Windows to the KDE desktop that Kubuntu uses is very easy to grip. To top it off, KDE is known to give pretty much many customizable options for the user, ranging from general ones to very specific ones. This is also considered something bad (not from me at least) because it gives you a LOT of options to customized. Don't be amazed if you find a customized option for your fridge there. Like Ubuntu, Kubuntu is not a lightweight distro.
Lubuntu - It uses the LXDE Desktop and, although not so much friendly as Ubuntu or Kubuntu, it is lightweight and fast to learn. This is for your old hardware that you want to bring back to life.
Xubuntu - Like Lubuntu, it is very lightweight but this time it uses XFCE Desktop. It is known that XFCE uses less resources so even in an incredible low hardware environment you might find this one pretty much light.
The 4 mentioned above are the main ones that focus on changing the Desktop for several types of users and needs. After this 4 come others that focus on the type of job or activity the user will be doing, for example:
Edubuntu - For education. Comes prebuilt with several learning programs. It is used for schools for example.
Mythbuntu - Used as a HTPC, it offers several programs (Specially MythTV) for TV recording and similar activities.
Ubuntu Studio - For multimedia users. It offers several preinstalled video editors, audio editors, graphic editors and any other multimedia editor needed for a multimedia workspace.
Ubuntu GNOME Remix - Is almost the same as Ubuntu with Unity, but instead use pure GNOME shell instead Unity as windows manager. This is the closest flavor that uses the entire GNOME set of programs and options. GNOME 3 from version 3.8 onwards require no Hardware Acceleration, hence it's less resources demanding than Unity and older versions of GNOME.
There are many others like Ubuntu for Android, but here I just mentioned the bigger ones.
List of "recognized" Ubuntu Derivatives:
The various different versions of Ubuntu differ only by branding and the default set of packages. On the inside all of the variations are similar and can be easily extended to include packages from each other just by going to the package manager and installing the packages you want.
So in a way there aren't that many differences between them.
On the other hand by default they focus on different things. Kubuntu focuses on KDE tools while Ubuntu focuses on Gnome tools and so on. So in the end it's mostly about your preferences.
Which one is most suitable for your old system depends a lot on how old the system is and what kind of hardware it has. The lightest ones of the bunch are lubuntu and xubuntu, not that others are too demanding either.
If I can, I would like to make a correction here. Ubuntu and Kubuntu receive official support from Canonical, the corporation behind Ubuntu. Xubuntu has not ever received support, although it is recognized as an official derivative. The developers of Xubuntu do so on their own time as volunteers. For use in an older computer, I would suggest either Xubuntu or Lubuntu, both of which are designed to use fewer resources than Ubuntu and Kubuntu.
Kubuntu, Mythbuntu, Xubuntu, Edubuntu, and Lubuntu are all based on Ubuntu but use a different desktop environment. Ubuntu uses Unity and Kubuntu uses KDE and ect..
Mythbuntu is a Media environment. Xubuntu and Lubuntu are genrally considered very lightweight compared to the rest. Edubuntu is an educational version of Ubuntu.