The Friends module is one of the social-networking features of Kentico CMS. It allows you to add the Facebook-style friendship functionality to your site. The module enables registered website users to request another user's friendship. When the friendship gets approved by the second user, the two users become friends. The process of requesting a friendship is described in Developer's Guide -> Modules -> Friends -> Requesting friendships.
Management of friendships can be performed either by the users themselves, or by the site administrators. Both options are described in the Friends management topic in the same chapter of the Developer's Guide. Users can manage only their own friendships, while users with appropriate permissions (described in the Security topic) can manage friendships of all users in the system.
The module comes with several web parts. Using these, you can add the friendship management functionality to the live site. All web parts related to the module are listed in the Available web parts topic.
By default, becoming friends brings just some improvements to the way the users can communicate and publish details about themselves, as described in the Examples of use topic. However, the friendship functionality may be leveraged largely in your custom code, enabling you to use these relationships between users for your specific purposes. The Friends internals and API subchapter should provide you with enough information to handle friendships in you code.