User Interface can be private (internal Salesforce users) or public. Public interfaces are provided to anonymous users using Sites - anyone could access a website built on Sites without having a Salesforce login.
If Sites is licensed, the functionality of Sites is available at Develop-->Sites.
A sites user has a unique domain name. One domain name can be used to support multiple websites. Example domain name could be - http://ap1.mydomain.salesforce.com/siteName . The domain name must be unique among all site users and can represent the organization's name.
Creation of sites requires specifying fields like Site label, Site Name, Site contact, Default web address, active site home page, inactive site home page.
Once an organization has registered a domain name with a domain name registrar, it can point the url to sites url.
Site template is a VisualForce page that defines the default look-and-feel of the sites page.
Force.com sites are
Public, unauthenticated websites
Accessed from branded domain names
Build with VF pages
From data and content in a Salesforce application
Creating a Force.com Site requires performing the following steps
Build a Force.com App
Design your web interface in visual force
Create a Force.com site
Activate the Force.com site
Key concepts about Force.com Sites security
Security for your site is controlled through public Access Settings
Access can also be restricted by specifying an IP range.