What is the difference between the groups "All Registered Users and Guests" and "Registered Users"?

Every user who is not registered and is accessing your portal while not being loggged into it, falls into the group "All Registered Users and Guests".
Every user that is registered and accessing your portal while being logged into it, falls into the group "Registered Users".

Both user groups "All Registered Users and Guests" and "Registered Users" are part of the standard setup of your portal and cannot be deleted - every registered user is automatically assigned to both groups.

Note:
If a portal is closed only users who are logged in can see any content, guests can only access the login page.