To me the help sections is pretty clear. See http://askubuntu.com/help/on-topic

Copied from there:

>We welcome questions about:

- Using and administering Ubuntu, including [official Ubuntu derivatives][1].
- Running third-party applications on Ubuntu.
- Development on Ubuntu.
- Services provided by Ubuntu
- Any question not mentioned below or here are great! There are no "dumb" questions!

>This is not the right place for:

- Linux Mint, Backtrack, Gnome-Remix (prior to 13.04) and other Linux distributions (try our friends at Unix & Linux Stack Exchange).
- Bug reports (How do I submit a bug?).
- Issues with the next version of Ubuntu (Ubuntu+1) (More information about Alpha and Beta release issues).
- Shopping Recommendations
- Support of versions for Ubuntu releases past "End of Life" (EOL)

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From the link: 

**Recognised Ubuntu flavours**

These are derivatives that use Ubuntu as their foundation and contribute significantly towards the project.

- Edubuntu — Ubuntu for education
- Ubuntu GNOME — Ubuntu with the GNOME desktop environment
- Kubuntu — Ubuntu with the K Desktop environment
- UbuntuKylin — Ubuntu localised for China
- Lubuntu — Ubuntu that uses LXDE
- Mythbuntu — Designed for creating a home theatre PC with MythTV
- Ubuntu Studio — Designed for multimedia editing and creation
- Xubuntu — Ubuntu with the XFCE desktop environment

  [1]: http://www.ubuntu.com/about/about-ubuntu/derivatives