Force.com sites provide the facility to build and deploy public websites and web applications using the Force.com platform. These websites and web applications can be directly integrated with the particular Salesforce organization without using a username and password of a particular user. Using the Force.com sites, you can build your site, which matches the look and feel of your company's brand, and through the Force.com site's page, you can publicly expose information relating to your organization.
There are no data integration issues in between Force.com sites and your Salesforce instance because sites are hosted in the Force.com sites. When you collect data from a Force.com site's page, data validations are executed automatically because pages are built using Visualforce pages. Users can access the site through a unique Force.com custom domain and the URL or registered own branded domain or subdomain that redirects to the particular site.