Public access settings control what public users can do on each Force.com site. To set the public access settings for your site: From Setup, enter Sites in the Quick Find box, then select Sites. Click the name of the site you want to control. Click Public Access Settings to open the Profile page for your site profile. More Info :- https://developer.salesforce.com/page/Authenticating_Users_on_Force.com_Sites
Public access settings control what public users can do on each Force.com site. To set the public access settings for your site:
From Setup, enter Sites in the Quick Find box, then select Sites.
Click the name of the site you want to control.
Click Public Access Settings to open the Profile page for your site profile.
More Info :- https://developer.salesforce.com/page/Authenticating_Users_on_Force.com_Sites