Groups Structure

referring back to my older queries regarding Communities, I have always assumed that a Community (our lingo) is a UserGroup (your lingo) that is managed by an assigned Moderator. Is that correct?

Yes and no. I ask for clarification when someone asks about groups because there's technically 2 forms of groups. User Groups and CB GroupJive Groups. CB provides moderation via CB Moderators, but you can't really assign 1 moderator over a specific set of users there just isn't any functionality for that at this time.

How about this scenario:
A senior moderator (Admin) creates a UserGroup called Group-A for GroupJive.
The Admin (owner) then invites members to join the UserGroup Group-A.
The Admin assigns a user as the moderator of Group-A.
The Admin repeats procedure for a new group, Group-B.

Possible?

Yes that's doable without having to do anything special and by just using CB GroupJive naturally. You can also force users into groups from backend or programmatically (e.g. CB Auto Actions) if you wanted.

If I am understanding this correctly, each GroupJive group would be like a community of users that share postings etc within themselves.

Correct, each group is its own isolated community. It supports full activity feeds via CB Activity and full galleries via CB Gallery so there's plenty of functionality to share within the group.