You put up a public web server to provide information to the world of Internet users. Anyone with a web browser and an Internet connection can attach to a public web site. You build a private web server as part of an intranet strategy to provide information to authorized users within an organization. A firewall is a server or router that puts restrictions on who can access a server or an internal network. It is an integral part of an extranet strategy.
Internal users can access both the private and the public web servers.
Another strategy is to make your public web server a stand-alone machine that is only connected to the Internet and is completely isolated from any internal networks.