A Visitor is A person who views the site passively and anonymously from the front end. 

Your primary objective should be to use the entire structure of management to serve your visitors; if visitors get something they need from the site, they will keep coming back.

Of course, you also probably want to get something from them, and the capacity to manage what visitors see and do through a system of membership and logging in enables you to establish a relationship with them, and potentially, for them to form relationships to one another.

The content that visitors see is controlled at the content-element level, based on a similar system of group and individual permissions to that applied to back-end users. Control of the display or any content element on any page can be limited in this way, as can access to information, downloadable files, and the capacity to edit certain content.