Manual List vs Rules-Based List
Manual lists do not automatically contain any members, instead you can add contacts:
Manually
From a Contact's Profile (make the list available in the Contact Manager first, in the list options)
By using Workflow or Action Group
Imported from a spreadsheet
Imported from another list (using the options at the top right when you have the list open)
Rules-based lists can be configured to automatically add contacts that meet the list rules
IMPORTANT: Rules-based lists will remove any contacts that don't meet the list rules when any of their information relating to those rules is updated. This includes:
Contacts that were added through Workflow or Action Group
Contacts Imported from another list
Contacts Imported using the Import tool
Manually added contacts will remain in the rules-based list until they are manually removed.