The four main desktop environments are GNOME, KDE, XFCE and LXDE. While Kubuntu and Xubuntu are considered official derivatives of Ubuntu, Lubuntu is not. Why is that so?
Update: Lubuntu has gained official status in 11.10.
At the spring 2011 Ubuntu Developer Summit, it was decided that Lubuntu was on track to become an official derivative with the release of 11.10. More information on the decision can be found in this 16 May 2011 email by Lubuntu developer Julien Lavergne.
As stated on their website, Lubuntu still aims to become a recognized derivative:
The ultimate goal of the lubuntu project is to earn official endorsement from Canonical.
Lubuntu is now an official derivative. :)
According to the Ubuntu website:
[Kubuntu, Xubuntu, Edubuntu, Mythbuntu and Ubuntu Studio] are derivatives that use Ubuntu as their foundation and contribute significantly towards the [Ubuntu] project.
Neither of these are official derivatives (according to this list, there are none), but they are recognized by Canonical (the company behind Ubuntu) as "contributing significantly towards the [Ubuntu] project".
Lubuntu and any other derivatives of Ubuntu, except those I already have mentioned, might use Ubuntu as a base for their distro, but they do not contribute, or at least not significantly enough, back to the Ubuntu project.