What is the difference between the *.ubuntu.com wikis?

From https://wiki.ubuntu.com/DocumentationTeam/SystemDocumentation

One of the major tasks of the Documentation Team is to take care of the documentation which comes with every Ubuntu system, and is available in Yelp, the Gnome help system. This documentation is also available on https://help.ubuntu.com.

In order to contribute to the system documentation, you need to know a bit about the tools and processes the team uses to maintain the documentation. Don't worry if you don't know any of our tools yet. They are very easy to learn and it's possible to make useful contributions before learning how to use all the tools.

The first thing you need to do is familiarize yourself with the docteam environment. This page briefly describes the various things you need to know and provides links for further reading and discovery.

From https://wiki.ubuntu.com/DocumentationTeam/Wiki

The Documentation Wiki is a sub-project of the DocumentationTeam. It is a community-driven and open area of the Ubuntu documentation website which anyone can edit.

also from here https://help.ubuntu.com/community/WikiGuide

This website is free for all to edit, and contributing is easy. You need an account for the website (see /Registration for details), and it is recommended that you read through this guide before contributing.

When should you contribute? Whenever you see something that could be improved or updated! Also, WikiToDo contains details of things that need doing. If you notice an error and do not want to correct it yourself, you can report it as a bug.

So yes, https://help.ubuntu.com is the official documentation but it is possible to contribute to it also. https://help.ubuntu.com/community and https://wiki.ubuntu.com are the same (if we are talking about the documentation of Ubuntu), they are the community maintained documentation. To see that the latter two are the same:

https://help.ubuntu.com/community/Installation

https://wiki.ubuntu.com/Installation

or

https://help.ubuntu.com/community/BinaryDriverHowto/Nvidia

https://wiki.ubuntu.com/BinaryDriverHowto/Nvidia

But https://wiki.ubuntu.com is not just about documentation as Braiam correctly pointed out in his above answer.

• https://help.ubuntu.com/ is not a wiki. Is the official documentation from the Ubuntu Documentation team. This documentation is akin to the man pages and is immutable through free ways.
• https://help.ubuntu.com/community is more akin to a serie of How-To's and straightforward guides for common actions users want to do.
• https://wiki.ubuntu.com/ is not only about documentation, it intends to be a broader site for any aspect referent to any part of the Ubuntu Community. In this you can find anything, from wiki pages dedicated to individuals (personal pages), information about releases, about any official team, and whatever is part of the Ubuntu Community.

I'm looking for clear/official/de facto descriptions of each wiki (or links thereto).

All pages has their descriptions except the wiki.ubuntu due it's broader scope. The help.ubuntu title reads:

Official Ubuntu Documentation

This site is where you can find the official documentation developed and maintained by the Ubuntu Documentation Project.

help.ubuntu.com/community/ says:

UserDocumentation

This user-created and maintained wiki is a reference for various Ubuntu-related Howto's, tips, tricks and hacks.

There's no limit to what source of information people should read. Some feels better reading the Wiki, some the official documentation, some the manual, some comes here. Is just matter of what people feels good about. They follow different ideologies and precept but they are not intended for a sole audience. What you want to do with the information given is up to you solely.