Simply, it's a public group that people can join that applies additional permissions onto your primary usergroup. So you can do everything your primary usergroup permissions allow, plus all the things the public group also allows.
If I setup a usergroup, the only way for a person to get into the usergroup is if an admin moves them there manually? Are there any work arounds, IE a membership usergroup that people can use a promo code to get into, or something?
In Vbulletin, I've got Public (Joinable) Custom Usergroups setup for specific topics. Example, I've got forums for Topic X. On my forums you cannot read the forums for Topic X unless you've joined and been accepted to the public usergroup for Topic X. Is there a way to do this in IPB?