An Internet site is a normal site that anyone on the Internet can access (e.g., www.msn.com, www.microsoft.com, etc.). You can set up a site for your company that can be accessed by anyone without any user name and password.
An intranet (or internal network), though hosted on the Web, can only be accessed by people who are members of the network. They need to have a login and password that was assigned to them when they were added to the site by the site administrator.