Recipients: Contacts in your admin console. There are three recipient statuses: Subscribed, Pending, Unsubscribed. They will only receive messages from you if their status is that of a "Subscribed".
Lists (or Environments): A List - also referred to as an Environment - is an independent set of messages, settings, statistics, filters, events, groups, subscribers, unsubscribed recipients, etc. You can create multiple Lists in the platform, as long as your account has purchased that feature and as long as your user has rights to do so. Groups are a static segment within a List. Therefore, Groups are always a subset of the recipients that exist in a List.
Groups: Groups are a subset of recipients (typically subscribers) within a List. Recipients can belong to multiple groups (similarly to “Categories” in MS Outlook). A group can be created, edited, or deleted at any time. The recipients that it contains will not be affected. When importing recipients, you can specify one or more groups that they should be assigned to. Groups are List-specific: they will not be visible in other Lists. Unlike filters, Groups allow you to create static segments of recipients. If a recipient is added to the Group "Subscribed on Facebook", the recipient will stay assigned to the Group until removed.
To segment your subscribers, use Groups. While importing contacts you can assign them to various groups.
Create different Lists, instead, to manage them separately. As said, a List is an independent Environment with its own set of messages, settings, statistics, groups, etc. and should be used for different type of communications (i.e. different products or clients for an agency). Use different lists if you communicate in various languages.
A common error is to create a list for each email campaign. Please note that creating a new list each time is time-consuming for you and potentially dangerous as the platform in this way does not recognize unsubscribed.
Managing Recipient Groups