The access class

It's the class that checks if the user has the right privilege to access a system functionality.
The privilege system works this way:
an user is associated with one or more user groups. Each user group has one or many permissions. The access to a content is checked against a specific or general permission.
So to establish if a user can have access to one functionality, the system checks if he belongs to a group which has such functionality.

There are some global permissions, instantiated in the core class. These global privileges are used when is not necessary to protect the access to a function in a particular way, but is sufficient to check for user authentication or user administrative role.