Important to note, if a subscriber unsubscribes from one group, he still remains subscribed in other groups he's assigned to.
Group vs List
A group is defined by the customized parameters and filters assigned by the user. A list is all recipientsAs defined in the Glossary, a group is a subset of contacts (typically subscribers) within a List. Contacts can belong to multiple groups (similarly to “Categories” in MS Outlook). A group can be created or deleted at any time. The contacts that it contains will not be affected. When importing contacts, you can specify one or more groups. Creating a group will only belong to one List and not across multiple lists.