What are the groups in /etc/group for?

http://askubuntu.com/questions/536818/where-are-groups-in-etc-group-docu...

I am searching the web but I cannot find where the various groups in /etc/group are documented, what they are for, etc (on a Debian/Ubuntu system, at least).
What are the implications of adding specific users to given groups? What additional rights would they get?
Is there a general convention amongst various distros about group names and permissions (i.e. how similar or different is /etc/group in Ubuntu vs. in Red Hat?)
Is there a generic way of finding out what a given group is for (beside guessing from the name)?
The primary reason why I ask is because I backed up /etc/group from an old installation and recently reinstalled Ubuntu, and I find many puzzling differences between the old groups and the new. I want to find out which to restore, and which users to assign to which groups.
But generally speaking, I am interested in existing documentation. The best I can find by searching the web is articles on user and group management, but none about what each group is for.

I have created the related wiki page and added a lot of information into it, especially with regard to how, ideally, things should be set up.