In CONTENIDO you are able to protect categories and articles from being seen by unauthorized visitors. To grant someone access she or he has to use a frontend user who belongs to a frontend group which has access to the protected category or the protected article.
Frontend Groups control the access to protected categories. Users need to be a member of a group in order to gain any access to any protected category or article. Every group has its own access rights. This is useful to provide different data to users of your website. For example customers might want to see different information from suppliers.
Users always need to belong to a group. They have a password with which they can log in to your website and then see the protected content.
Access Control in Modules/Plugins
There are quite a few classes that relate to the fontend users and their permissions. The function to check if a group has access to a certain action is in the class cApiFrontendPermissionCollection.
Have a look at the other classes: