We're still working on this section of the documentation, but it'll be ready soon!
As defined in the Glossary in the Getting Started section, a "recipient" is any contact in your admin console. There are three recipient statuses: Subscribed, Pending, Unsubscribed. A recipient will only receive messages from you if their status is that of a "Subscribed", and, therefore, should never be confused for a "subscriber" unless assigned that status.
Groups can be created to section recipients based on any type of criteria: sending activity, profile demographic, filters, etc. to create more targeted messaging. 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.
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. 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.