What is the main difference between Ubuntu, Kubuntu, Mythbuntu, Xubuntu, Edubuntu and Lubuntu?
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 TV - Built for TVs, it helps you manage TV content in a very intuitive manner. Very easy to learn.
There are many others like Ubuntu for Android, but here I just mentioned the bigger ones.
List of "recognized" Ubuntu Derivatives:
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.