What is the difference

On juvenation, between JOINING a GROUP and SUBSCRIBING to a GROUP?

And what causes some "forums" to be directed to my personal email and others not to?

I can answer #2 if look on a forum page there is a box that says "E-mail me replies to this post." on the bottom of the page. If it's checked you will get e-mails. Just uncheck if you don't want the e-mails.

subscribing to a group keeps you updated with that group's activity.  You'll get notifications for it.  Where as joining a group allows you to participate in the activity but not necessarily get the notifications to all of its activity.  It's more of a preference if you want to follow a certain group more regularly.