There are three kinds of basic lists (determined by the setting of Allow Postings By on the List Security page):
Anyone (open list). Anyone can post a message to the list; the individual that posts to the list does not have to be a list subscriber.
Subscribers. Only a list subscriber can post a message to the list.
Moderated. Only a list owner can post a message to the list. Moderator is used when you want the list owner to review all messages before they are posted to the list.
You can further restrict the message posts with:
A password requirement.
Those who send mail to a list restricted by a password and/or posters' list will have their mail returned with a "Restricted Post" message.
Note: If there are only a few individuals who the moderator wants to allow to post to the list, the moderator can give those few the appropriate password. However, if there are more than a few individuals who are permitted to post to the list, it may be more efficient to use a posters list.