[ The previous revision of this question didn't get much response and was incomplete because it did not include the derivatives, at least one of which also has editions. I have revised it to hopefully be more complete and I have switched it to a community wiki. ]
Note: If you have a suggested set of tags to identify the various Ubuntu editions and derivatives I would recommend posting them as answers so that they can be voted on and discussed.
Current usage on Ubuntu.com
On the official Ubuntu website the term "edition" is used to indicate the various formulations of the main distribution.
In the past Ubuntu Netbook was also known as the "Netbook Remix" but Jorge Castro noted that this usage has been officially dropped in the latest releases.
The official website divides the known derivatives into several groups:
- Recognized (significant contributions to the overall project)
- Localised (support for languages not included in the main CD release)
- Other (see the derivatives wiki for more lists)
It seems reasonable to limit this discussion to the main Ubuntu distribution and the recognized derivatives. Based on the derivatives wiki an attempt to be more inclusive could easily become quite confusing. The recognized derivatives are:
- Kubuntu - Ubuntu with the K Desktop environment
- Xubuntu - Ubuntu with the XFCE desktop environment
- Ubuntu Studio - Designed for multimedia editing and creation
- Mythbuntu - Designed for creating a home theatre PC with MythTV
- Edubuntu - Ubuntu for education
According to the project website Kubuntu has two editions:
- Kubuntu Desktop (usually just "Kubuntu")
- Kubuntu Netbook (also known as "Kubuntu Netbook Remix")
The Edubuntu derivative doesn't appear to have editions in the same sense as it's siblings but that project does produce several "application bundles" including one that is intended for installation onto a Kubuntu-based system.
Current usage on the Ubuntu Stack Exchange
The desktop edition appears to be mostly tagged as ubuntu-desktop has been deprecated.
We assume questions on the site apply to the Desktop edition unless otherwise tagged or it is obvious.
There is also desktop but it's unlikely these refer to the Desktop edition, more likely the concept of a "desktop" (desktop background, etc).
These are tagged ubuntu-netbook with some tagged unity.
ubuntu-server redirects to server
At the moment, Kubuntu Desktop questions are tagged with either kubuntu or kde (and both in a few instances).
There are a couple of questions tagged knr. Most questions tagged netbook are referring to the main distribution (as noted above).
Questions relating to the XFCE-based derivative are tagged with either xubuntu or xfce (and a few with both).
Questions regarding the home theatre derivative are tagged with mythbuntu. However, there are more questions tagged with mythtv. The former should probably be a synonym of the latter.
There are a small number of questions with edubuntu
There are a very small number of questions with ubuntu-studio