This is a great question - ideally the difference should be indicated right in the app
Where am I?
In Tribe you can ask and answer questions and share your experience with others!
Groups are like another feed but you post things within the group's topic.
Tags are topics that don't get their own dedicated page.
And topics are tags but they get their own page.
I bet someone can do a better explanation than I do.
I also don't really use groups for my network.
I hope it helped somewhat.