Craft pieces accessed by people on domain only
The back end of craft (admin log in section or Control Panel) can only be accessed by admins and users that an admin has added. The front end will always be visible to everyone while the backend will be password protected. 

If you want separate front end displays for people who have usernames and passwords and those who dont, you could add a front end log in form:

http://buildwithcraft.com/docs/templating/login-form

And display one set of templates for those who are logged in and another for those who arent logged in via conditional statements e.g. 

{% if currentUser %} 
  Protected info 
{% else %} 
  public Info 
{% endif %} 

If this isn't quite the answer you're looking for could you be slightly more specific with what you mean by "on the domain". Describe exactly what you're looking to achieve.