Each person can be a member of more than one group.  To see which groups you belong to, look in your profile:

When sending an email through the website, a secretary selects one of these groups to send it to.

When you receive an email it has an unsubscribe link at the bottom.  If you click on this link, it unsubscribes you from emails for just that group (the group that the email was sent to).  It does not remove you from the group.

If you unsubscribe, but keep on getting emails, it is probably because you are a member of more than one group.  In this case you can either go through and unsubscribe again from the new email that you received, or you can remove yourself from groups using the blue gear icon shown above.