Yes web pages quite often have private and public areas.
The private pages are sometimes referred to as an intranet as it is part of their network that is only intended to be used in the organization. Organizations will post on the private pages information needing to be used by people in the company but they do not want outsiders to see. Things that may be on a private web page are wholesale pricing sheets, leave forms, lists of clients, proprietary information about products.
Another use of private web pages could be areas where employees update information on the public web page.
Access to private web pages is controlled by passwords. The creator/manager of the web site can set passwords on different pages (and they can be different by page) and then the password s are only given to those who are authorized to visit those pages. When they go to a private page it will ask for the password and sometimes more information depending on security needs.
Although the World Wide Web on the Internet is an open internet you can still secure items from people viewing. However with all computer security when stuff is on the internet or on a computer connected to a network connected to the Internet whether on public or private pages it is impossible to 100% secure the information.