Group vs List
As 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 and the contact information that it contains will not be affected.A group will only belong to one List and not across multiple lists.
Managing Recipient Groups
Groups can be created to section recipients based on any type of criteria: sending activity, profile demographic, filters, etc. to create more targeted messaging. The Manage Groups page's functions include: search for a group, delete a group, delete recipients from a group, delete both group and recipients, add a new group and rename a group. From the Modify Group Assignments tab at the top, you can even reassign or move recipients from one group to another.
Important to note, if a subscriber unsubscribes from one group, he still remains subscribed in other groups he's assigned to. However, when importing recipients to a List, you can assign them to multiple groups at that time.