A Role can be associated with Modules and actions.
For example, the Gallery module has the actions create, edit, list, view, upload_images, ...
The Userprefs ("My Account") module has the action set_password.
Now you can associate the role 'guest' with
and the role 'admin' with
and the role 'user' with
Now a normal user would get the roles 'guest' and 'user', and the admin user
the roles 'guest', 'user' and admin'.
Edit the permissions of role "editor":
Click on "Add":